I'm a Molecular Biologist/Musician/Programmer. I use an SGI and Macs at work, and I write software for Macs at home. My wife uses a PC. For music (which nobody seems to have mentioned) Macs are far better than PCs. Even the heavyweight software which is available for both, e.g. Cubase works far better on the Mac. There's a much larger selection of general software for PCs, but the quality of Mac software is higher. As for the user interface - use whatever you like best, and if you prefer you can run Linux on either.

Best ABC software is BarFly but then I would say that, since I wrote it:-)