The R-690 is not a cage I would recommend, its narrow, and awkward to get into to decorate or clean, the ramps are steep so not great for an aging rat. If you are going to get a good Martin's go for the R-680, powdercoated. . Its a much easier cage to decorate, clean, etc. I cannot get Martins shipped to Canada anymore but still have an R-680 here that I love.
R695, powder coated with flip top lid is a great cage …… cover full level with a piece of thick linoleum cut to fit and balconies with a dollar store dish drain tray cut to fit and attached with zip ties . Assemble cage using 100s of zip ties because the c-rings will rust
I've had a single unit critter nation, Martin 680, and Martin 695. And I think Martin's are great cages!
I wouldn't go for anything smaller than a Martin 680. The 680 is a great cage. Nice space for 2 rats and east to decorate. However, if you're willing to spend a bit more I think you might as well go for the 695. It is only $40 more and much more room and levels for the rats! Another thing I like about Martin cages(at least for the 680 and 695) is that they are SOOO much easier to transport than the critter nation. I'm able to lift my 695 into the shower for a deep clean. The cage also comes with a nice pan that keeps bedding in. I also think it is easier to attach things wherever you want compared to the critter nation. And the doors are a nice large size.
Overall, they are really great cages. The only things I like less about them than the critter nation is the smaller doors and that they aren't as pleasing to the eye.