I know this is an old thread, but I just came across it. I have both the micron and infinity and use both quite a bit. The micron is my daily driver for most things. I think it interesting that some find the response time of the Infinity slower than the micron. I've found quite the opposite, but it may all depend on what kind of paint you are throwing, mix, air pressure and so on.
I use the infinity 0.15 for really fine work. The micron in my opinion is a much better all around brush, but for pulling very fine detail without losing paint flow or getting spotting, the Infinity beats it in my hands. Once you open up however and for general casual use, I find that the micron gives me a wider range of control. The infinity opens up too quick if you go a bit bigger, and the lack of something like a MAC valve to control the pressure at the gun mean, that while still good, you don't have quite the control you do with a micron. But for extreme detail and working close to the canvas, I much prefer the H&S Infinity 0.15.
I've wondered about this over the years, as the micron 0.18 is very close. I think the difference ultimately boils down to something about the taper on the needle, and with the same taper on an Iwata needle, I bet they would perform next to identically at the small scale.