I used to own the A500, and it's an excellent can. Here's a thread that went in depth about the difference between A900 and A500: http://www.head-fi.org/forums/showthread.php?t=95224
Based on it I went with A500 at half the cost, and haven't regretted it. At some point I may yet get back in the A-T game and get an A900 LTD, though it's not for the A500 lacking anything, but I just want to play with a different set of cans :-)
Keep in mind though, the way you asked the question, most will say A700 since there is no tradeoff implied. But you may want to consider using the saved cash on the A500 to get an amp or upgrade other components.