Ok I just checked some benchmarks of CPU on that MBP (i7 2720QM) and to be frank and it's straight forward it's actually pretty damn powerful.
It's actually as good as the desktop i5-2400 CPU with even some slight advantages over it. Single threaded and multi threaded performance both rocking sky high for a laptop. Damn, how the heck Intel pulled this off with half the TDP of i5-2400 on the same year both of these CPUs were released I have no idea. No wonder those MBPs equipped with i7s were so expensive back in the day.
If this is gonna be your main and only computer/powerhorse, definitely get MBP, otherwise..
What kind of graphical work are we talking about?
See, if you already have a powerful Desktop of some sort, getting MBA would make more sense due to battery life, SSD (very fast for browsing, turning on and off, writing, casual programming or launching non-cached into RAM apps), weight etc...
But if you are NOT gonna use it as your secondary portable computer yet instead will heavily rely on it to do every single task, get MBP and upgrade its HDD with SSD in the near future. You should probably expect to replace the battery in a couple of months or so too if it hasn't been replaced yet.
I kinda got curious so please do update us on your decision.
PS. You didn't mention it, but you do know that 2011 MBP equipped with i7 only comes with bigger 15inch screen, right? So naturally it's much bigger laptop than 13" MBA.
EDIT: In previous posts I didn't realize it's a 15inch model we are talking about so I was in fact wrong about the graphics.
Unlike 13" model, 15" MBP comes with Radeon HD 6490M, so graphics performance is not bad.
Which leaves MBA superior only in storage department (which can be upgraded on MBP) as far as performance goes.