I like Martin's cages. They are cheap for what you get. My only complaint is that with some models, the doors are too small for me to wedge my body into for cleaning, or pinning shelf liners. They are really good about working with you for any kind of customization you'd want though. I know a breeder who got a custom rat cage from them whose dimensions were essentially their RUUD model turned on its side. It was amazing. I would recommend only certain models of the Martins for your rats. Some that they offer are only appropriate for juvenile rats. Some are not appropriate for rats with mobility issues. Just as a general rule of thumb, nothing narrower than 18 inches deep will work for full grown male rats. The 14 inch deep ones can work for the smaller adult females, but they will need a lot of climbing room. Also, the ramps of the narrower and taller ones can be too steep for older rats.
Also, though they offer cage toppers, I find them to be a waste of money because the rats tend to not use the tank level at all, and only stick to the top levels.
I currently own a double Critter Nation. You can get them as a single or a double. On the downside, they can be expensive and heavy to maneuver. On the up side, they are HUGE, and easy to clean and decorate. You'll occasionally get really good deals on them. I brought home mine last fall for @225.00. The best deal available currently that I've found for a brand new one is here: http://www.allpetfurniture.com/Midwest-Pets-162-MDW1242.html
They also have a single unit for cheap as well. The sale price is comparable to what you'd pay for a similarly sized Martins (including S & H), and you'll get a lot bigger cage footprint for your rats.