You might want to try the touring forum for rack advice, the readers/posters there know more about racks.

Generally, there are 2 types of racks "mountain" for 26" wheels and those built for 700c wheels/tires.

I like the Jannd Expedition rear rack because it has a nice wide platform and holds the trunk bag very securely that I carry most of the time.

One thing to consider is if you decide to use panniers you need a combination of chain stay length and rack positioning to make sure that you don't hit the panniers with your heel each time you pedal (heel strike).