Evo has a couple of slightly higher specs, LCD versus AMOLED, the option of connecting to a Wimax network where available, and cheaper plans.
Obviously check coverage in your area on Sprint and Verizon's site. If coverage is good for both, Sprint costs less. If you get good Verizon coverage and poor Sprint coverage, no amount of savings will make up for that.
My general rule of thumb is to check coverage first, then out of the carriers that cover your area, look at plan price second (don't get fooled by base price when data, text, nav, etc. cost more to add on), and once you figure out who has good coverage and cheapest plans, choose your handset.