Software to emulate Windows 7 on OS X, for developing
I'm a Fusion user and have been for a few years now. Fusion 4.x has been a nice upgrade and it's a very stable way to run the latest Windows OSes on OS X. I think you'll be fine with Fusion or Parallels -- both are tested and support Win7. VirtualBox is a nice option, but be prepared to go it alone if you run in to any issues.

None is really better than the other in my opinion. They all have some overlapping features and they all have some features that are unique to the product. Try before you buy if you can.

I will comment on your system: you're going to find things running slow with only 4GB of RAM and a Win7 VM for development work. Visual Studio is a pig and it's running on top of an OS that's not terribly great at memory management. I'd recommend you upgrade that 4GB to 8GB (or more if your model supports it) if you want the best virtualized OS experience.