I opted for the 512 so I can do Windows in Parallels. However, 256 would have worked too. I only have 60gb dedicated to W7 now, but I may need more in the future (which is why I went for 512).
Almost exactly my situation. My Parallels 9 Windows 8.1 installation takes a bit under 40 GB, including a fair amount of software installed in Win 8.1 (MS Office 2010 and a few other programs (Adobe Acrobat 11, etc.)). The pretty well-populated OSX 10.9 installation is ~200 GB of "stuff" (programs, photos, iTunes content) leaving the 512 GB SSD a little less than 1/2 full. If I weeded-out some of the excess "junk" in OSX, I could live in a 256 GB SSD with at least the desired 20% free space.
BTW, virtual machine (VM) software takes a bit of the hassle out of using Windows -- no need to partition the main drive, no need to deal with getting Bootcamp set up, and no need to have to shutdown and restart every time you want to switch OS's. Also, cross-OS file sharing (at least under Parallels; I don't have much VMWare experience) is easier.
Having used both BootCamp and Parallels, the only advantage I see to BootCamp is that it is "free" (if you don't count the cost of your time in installation and day-to-day use). For me -- haven't been a starving student for a number of years (that is now my kid's job
🙂) -- the cost/benefit ratio is a no-brainer: definitely worth the money for good vm software. And knowing what I know today, even as a student I would go for it, where at least Parallels has a reduced-price student license (full software functionality, not a minimum software installation).