I have both an HP-TH and an Eclipse with both a .5 and a .35 setup, so I can answer that.
The HP-TH has better atomization. The Eclipse in both .5 and .35 configurations does finer lines.
The HP-TH cannot do fine lines because the needle/nozzle taper is too blunt, so the distance required to do fine lines is shorter than the height of the nozzle cap. It's a brush designed to spray a wide cone at high volume for large scale coverage. It has better atomization than the Eclipse because the airflow in the head is better/more precisely engineered.
The difference between the .5 and .35 setups in the Eclipse is mostly in how they handle. There is no difference that I can detect in atomization or max paint volume. The .5 lets you spray paints with larger or more irrgular pigment particles, at the cost of the trigger travel between "fine line" and "full blast" being dramatically shorter. Both can do equally fine lines, but the .5 requires finer finger control to find and maintain the trigger sweet spot for a given line.
I have 2 Badger SOTARs which have smaller needle/nozzles with a steeper taper than the Eclipse. They have finer atomization the the Eclipse, but not as fine as the HP-TH. They do not do finer lines than the Eclipse, but the lines are crisper, and the trigger travel needed to find and hold the sweet spot for a given line is more forgiving.
I have 2 Paasches, a VL and an H, niether of which can do fine lines because the airflow cone is very "soft" regardless of pressure, resulting in spray patterns with shallow, fuzzy gradients. If I try to do fine lines, I get thick fuzzy lines that are basically all overspray and no core.
In my experience, anything with a needle taper which puts the apex of the paint spray far enough out to be acessable can do fine lines, as long as atomization and airflow cone is "good enough", and your hand/finger control is good enough to hold things in the right position. If atomization is really bad, the lines will have a granular look instead of smooth. How crisp the line edges are has to do with the airflow pattern: anything with passable atomization can do crisp lines as long as the airflow cone has a crisp boundary. This requires careful design and precise machining, I should think, and so would be likely to coincide with finer atomization as brushes go up in quality, even if the two are not necissarily directly linked.
Conversely, it is entirely possible to have a brush which produces very fine atomization, but does not produce crisp lines because the airflow cone has too "soft" a boundary. Such brushes would be able to do fine lines, but the lines would always have "fuzzy" gradient edges instead of being crisp.
Steeper/longer needle tapers result in a longer, narrower airflow cone, making trigger travel and tip distancing more forgiving. This makes it easier to control fine lines, but does not necissarily mean finER lines.
So brush X being objectively able to do finer lines than Brush Y would mean a combo of:
1: A "good enough" minimum of atomization fineness/quality, in which droplet size and spacing is no greater than X% of the target line width, and
2: An airflow cone which is crisp enough to keep paint spray confined more tightly, resulting in a spray pattern with a steeper gradient at its edges.
Other elements like needle taper and minimum air pressure make fine lines easier, but not more or less possible in a binary sense.
Provided those two elements are in place, the brush can do equally fine lines... for a given value of "fine", naturally. The tinier the line, the more restrictive those variables have to be. However, my Eclipse can go hair thin with either setup, so IMO a "better" brush is one which makes it ergonomically easier (a steeper needle/nozzle taper), or which produces better gradients ( tighter air cone boundaries for crisp lines, finer atomization for blended edges).
The HP-TH has better atomization. The Eclipse in both .5 and .35 configurations does finer lines.
The HP-TH cannot do fine lines because the needle/nozzle taper is too blunt, so the distance required to do fine lines is shorter than the height of the nozzle cap. It's a brush designed to spray a wide cone at high volume for large scale coverage. It has better atomization than the Eclipse because the airflow in the head is better/more precisely engineered.
The difference between the .5 and .35 setups in the Eclipse is mostly in how they handle. There is no difference that I can detect in atomization or max paint volume. The .5 lets you spray paints with larger or more irrgular pigment particles, at the cost of the trigger travel between "fine line" and "full blast" being dramatically shorter. Both can do equally fine lines, but the .5 requires finer finger control to find and maintain the trigger sweet spot for a given line.
I have 2 Badger SOTARs which have smaller needle/nozzles with a steeper taper than the Eclipse. They have finer atomization the the Eclipse, but not as fine as the HP-TH. They do not do finer lines than the Eclipse, but the lines are crisper, and the trigger travel needed to find and hold the sweet spot for a given line is more forgiving.
I have 2 Paasches, a VL and an H, niether of which can do fine lines because the airflow cone is very "soft" regardless of pressure, resulting in spray patterns with shallow, fuzzy gradients. If I try to do fine lines, I get thick fuzzy lines that are basically all overspray and no core.
In my experience, anything with a needle taper which puts the apex of the paint spray far enough out to be acessable can do fine lines, as long as atomization and airflow cone is "good enough", and your hand/finger control is good enough to hold things in the right position. If atomization is really bad, the lines will have a granular look instead of smooth. How crisp the line edges are has to do with the airflow pattern: anything with passable atomization can do crisp lines as long as the airflow cone has a crisp boundary. This requires careful design and precise machining, I should think, and so would be likely to coincide with finer atomization as brushes go up in quality, even if the two are not necissarily directly linked.
Conversely, it is entirely possible to have a brush which produces very fine atomization, but does not produce crisp lines because the airflow cone has too "soft" a boundary. Such brushes would be able to do fine lines, but the lines would always have "fuzzy" gradient edges instead of being crisp.
Steeper/longer needle tapers result in a longer, narrower airflow cone, making trigger travel and tip distancing more forgiving. This makes it easier to control fine lines, but does not necissarily mean finER lines.
So brush X being objectively able to do finer lines than Brush Y would mean a combo of:
1: A "good enough" minimum of atomization fineness/quality, in which droplet size and spacing is no greater than X% of the target line width, and
2: An airflow cone which is crisp enough to keep paint spray confined more tightly, resulting in a spray pattern with a steeper gradient at its edges.
Other elements like needle taper and minimum air pressure make fine lines easier, but not more or less possible in a binary sense.
Provided those two elements are in place, the brush can do equally fine lines... for a given value of "fine", naturally. The tinier the line, the more restrictive those variables have to be. However, my Eclipse can go hair thin with either setup, so IMO a "better" brush is one which makes it ergonomically easier (a steeper needle/nozzle taper), or which produces better gradients ( tighter air cone boundaries for crisp lines, finer atomization for blended edges).