Thinkpad T420 / T520 - differences "in detail"?

[hirnsaege]

Dear readers,

as my beloved Thinkpad R60's GPU/graphics memory died yesterday (can run at VGA resolution, everything else results in bluescreen), i'm looking for a replacement, that would be a T520 or T420.

My use case:
-- I'm a Graphics Designer. This will be my only computer (syncing never worked for me). I use it to visit customers, and have lots peripheries at home, connected to an advanced mini-dock for fast leave and return.
-- Mobility isn't *very* important, so the 520 could do the job. But if 420 is equivalent in the rest, it would be better.
-- Screen: resolution will most likely be 1600x900px, this seems to be the most useful to me. Display quality is nice, but not overly important (having a calibrated monitor at the office). as i read, 420 screen is worse than 520 - does this apply to the 1600x900 too?
I'm going for a nvidia discrete graphics version, as it's the only computer i own, and i want some 3D capabilities at least.

What's important are peripheries:
-- i want to connect at least one DVI monitor to the notebook or the dock - better would be two. analogue VGA connection doesn't do it for me and my displays. is this possible with the discrete nvidia graphics chip, or are there limitations in 420 and/or 520?
-- as i need some external storage, the faster the better. i'd like to have SATA3 if possible in the primary and(!) in the drivebay, i'd like to have eSATA at the dock and the notebook. USB3 seems to be out of reach at the moment – would have liked to wait for that ... differences between 420 and 520?

more questions:
-- Would you go for a slow quadcore (i7, 2.2ghz quad) or a faster dualcore (i7, 2.7ghz dual)?
-- Guess i can't use my old advanced mini dock, but go for the new minidock 3 or minidock 3 plus?


so after all, the main question is: is there something the T520 can do, and the T420 can't, that would be relevant to me?

thanks for your help
hirnsaege