If I remember correctly they are exactley the same except the 601 gets a pwoer boost on the amp and to my knowledge and research they should be the same price just that the 601 is the 2003 model and the 600 is last years model!
Also, the 600 had some quirks. Some units would just lock up altogether until you unplugged it and plugged it back in, at which point all your settings were gone (which becomes annoying after about the third time). Most notably, I found a fairly common bug with the TX-SR600 in which out-of-phase sound in the left and right surrounds was being thrown out when in DD-EX mode. Onkyo is aware of the problem and has a fix, though I don't know how much this problem has been publicized. I got two brand new units that had the EX decoding bug and Onkyo finally had to send me a pre-tested unit. People at the Spot are having good luck with sending their 600's in for service to get this corrected, though if you're buying a new receiver you may not want to go through immediately having it serviced.
In other words, go with the 601. Now that Onkyo knows how to test for the EX phase decoding bug, I would think that new models won't exhibit this problem. Besides that, the 601 has a little more power and better Zone 2 capability (if that's important to you).
I think the 601 also has a video combining circuit that will convert SVID to composite and vise-versa, that way you can hook everything to the tv with svideo cable and still watch your vcr that is only composite out. Course depending on your tv (like in my case) this isn't a big deal...