Even though I've been using the 2nd for the first year of med school, I just upgraded to the 3rd edition. I actually like it a lot - a lot more pictures and such. The pages are a little bigger and there are occasional blue boxes that aren't in the 2nd edition that just explain things in a little more detail. I think it helps more comprehension.
If it's a matter of money only ($35 vs. $10), it's a no brainer: go for 3rd edition.