I need an honest answer.
I need to install Windows via Bootcamp on my Mac.
I will be using it mostly for visual studio.
Which is better for me - Win7 or Win8?
Macbook Pro or desktop? Win8 isn't officially supported by bootcamp yet, you can install it but some drivers don't work. You have to go track down a 3rd party driver for trackpad, if you google it there are multiple guides out there to get it set up properly. If you don't want to deal with the trouble win7 is easier to install right now. win8 will be supported sooner or later though. I'm not sure but if you need to purchase windows license, 8 is cheaper and easier to get too. the multi touch track pad on a mbp make using windows 8 UI and gestures a lot more easier too.
As far as using Visual Studio goes, there is no advantage with either, maybe things run a bit faster on 8 and the new Visual Studio with new UIs is consistent with rest of Win8 look if that matters to you, I imagine not. If you care about developing software and apps for the new Windows environment you have to be on 8 if not then it wouldn't really matter.
other than these things it wouldn't really matter which way you go, functionality is same in desktop. you only have a new start screen.