What kind of stuff are you wanting to do with it? There are fans of both brushes, so I would say you would be happy with either.
I can't speak for the infinity, but I love my HP-CS, it really is a reliable, consistant, hardwearing, easy to maintain and clean brush. Not strictly a detail brush at .35, however it does do really, really nice fine detail work, and the nozzle size is slightly more forgiving for paint mixtures than some of the fine detail brushes. It was my only gun for around 2 1/2 years, and I used it for eveything from paintpals (5 x 7 inch) to automotive, though I have never really painted anything bigger than A4 size, but loved the fact that it had the versatility to do larger areas too. I always recommend this gun to learners as it's great to learn on, but the better you get, the more you can get out of it, you won't outgrow it later on....although if you are anything like many members you will end up with a collection Lol,....but this gun will be there for the long run. I now have a micron, but still use this too, and between the two will never need another brush. I think the hp-cs is the best gun for the money, and you won't be disappointed with it.
The infinity brush is a detail brush, and I believe comes with different nozzle sizes, so you can swap them as needed (just don't mix them up, I remember a couple of people having problems with that). I have heard they perform great, but never having used one, have no idea about reliability, etc, but I'm sure someone can fill you in on that.