I have both the regular Sotar, and the Slim. Well more than just those two, but that is another story... I would make the call for the regular Sotar ahead of the Slim, for the simple reason that you CAN add more color at the start if you want to. The Slim will really only hold 3 drops of paint at a time, which is not a lot. For doing fine detail, more than enough, but when trying to paint a 12x12 square with base color, well, you will be filling a LOT. On the other hand, there is nothing that says you need to add more than 2 or 3 drops to the regular Sotar to work... Just that you can if you want to.
If you want to expand the search into other possible brushes, that is another can of worms - and I would want to know a good bit more about what and how you are painting, and with what paint, etc...
This seems self indulgent but maybe it will help: I am just painting pictures. I did a solid 9 months of daily hours (at least three to four hours every day), of practice with an airbrush: dots, lines, daggers, and shading. But I stoped due to my abusive wife. It is a stupid story. When I just picked up the airbrush again, it was almost like no time had gone by. I figure I just did so many of those practices, that they became fused into me.
I like to do abstracts a lot. I like to use symbolic icons. Sometimes people ask me if I am a tattoo artist. I like to abstract things. I like painting flowers because I can either paint one that looks like a particular flower or abstract it as much as I like.
There is bunch whimsical-ness (and what could be called) psychedelic elements in what I do. Never really dark themes. If you can dig a music example: think more Phish less Grateful Dead. I really dig the zen doodle stuff. Lots of lines and patterns. I love patterns and textures.
I would like to get more into line drawing. Sometimes I use line drawing techniques for shading and depth. Hence the need for a brush for fine detail.
I love working with watercolors. I would like to try watercolors through my airbrush.
I love using ink. I have done some Japanese style ink painting.
I really love using color pencils to get fluid with color. I like how I can get subtle differences by building up color with the pencils. That way I can get color to ebb and flow yet meld seamlessly together. I just takes too long.
Right now I am just using acrylics through the airbrush. I am not a big fan of acrylics but I like them through the airbrush. I think I would be happy using ink and watercolors.
I have no idea if that helps at all. If you need anymore information, just let me know.
(Since I used a music example, a little background information: I am a trained/schooled simi-professional (or just professional, depending on how you define the terms) musician. Before finding music, I wanted to be a visual artist. I still remember my day dreams if that. However once I started playing music I was hooked. I always did art and was nicknamed in junior high due to my drawings)
