Would you be taking your laptop to uni on the bike? If so a large laptop is a definite no-no. A 13" MacBook Pro (or similar size) would be fine, but a MacBook Air might be better, if you want to go with the Mac.
On the downside the Mac won't run some software that you can run on your Windows machine. What software do you need for your PhD? That should be a factor. Word processing is easy on the Macs, either with Microsoft Word, or things like Apple's own software Pages. Ditto for spreadsheets. But if you are using for example statistics software that wouldn't run on the Mac, except under Windows emulation. Oh and you can get EndNote for the Mac.
I used a Mac throughout my part-time PhD. Near the end I got a 15" MacBook Pro which I wrote all my thesis on, very happily. I now have a 13" MacBook Pro which I'm very very happy with.