Personally, I would say Mac. I don't think either one is going to make you a better editor or render anything faster, but here are my reasons for choosing Mac:
- Can work with all Adobe products AND Final Cut Studio
- Seems to be ubiquitous with the mid -to- high level editing companies/organizations/institutions that I've come into contact with.
- I really like Final Cut Studio more than Adobe Creative suite for video postproduction. It could easily just be me, but I've found Final Cut to be more stable, intuitive, and faster than Premiere, plus a heck of a lot more powerful. Shake/Final Cut Pro vs After Effects/Premiere Pro will never be a competition in my mind.
- You get all the power of UNIX. This might not matter to you if all you're looking at doing is basic editing, but I use the command line every single day to help me out, mostly with media organization, but just earlier today I wrote a command line utility to help me out with a visual effects shot I was doing.
When I started film school, I was among two of thirty people in my class that used a Mac. By the end of the first year, 20 of them were Mac users, and by the end of the third year, every single one had switched.
Do with that what you will.
-edit-
What kind of video you're editing and in what format will define your hardware requirements. I would say don't use USB at all, even for SD footage. Firewire will get you through SD (even ProRes uncompressed) and things like HDV. For uncompressed/HDCAM footage and 2k/4k RED, you're going to need a separate, dedicated SATA drive from your boot drive. This necessitates a tower over a laptop/iMac setup, but most towers can support 4-6 internal SATA drives.... plus, they're cheap ($100 for 1 TB).
RAM's another consideration. You're not going to get away with less than 2 gigs. I have 6 and I still run into issues when working with VFX/realtime filters on uncompressed HD and RED 4k. Things to keep in mind.