Go for the R-685 for sure. The R-690 has limited space for toys/hammies and very steep ramps for when your rat is older and feeble. You need to modify the cage then. My friend was debating between the R-680 and the R-690, got the R-690 and didn't like and ended up giving it away. she said she would never put more than 2 rats in there.

I took one of my R-680's and modified it into an R-685 for my older geriatric gals. I have 5 in there now, but would normally only put 4. I have 2 older girls who aren't going to be around much longer *sniff*