for a trip of that duration you will be carrying a lot of weight of canisters and especially empty canisters. A white gas stove would be more effecient weight wise.
I respectfully disagree with
Ed_T regarding wood burning stoves. I am not an advocate of wood burning stoves. I know some people still use them, but there are a lot of draw backs associated with them; soot on your pots, dealing with smoke, impact on the environment, gathering firewood, constantly attending to a fire made of small twigs to keep it going. If you use a wood burner inside your tent you have to give up some room inside the tent. You can avoid all of those disadvantages with a white gas stove.
The MSR Whisperlite is probably the 30-06 of white gas stoves.
http://cascadedesigns.com/MSR/Stoves/Fast-And-Light-Stoves/WhisperLite/productYou know on a 20 day trip with four people, you're gonna want to have two stoves. You can prepare dinner twice as fast with two stoves vs one stove and you will want to have a backup in case one stove craps out on you.
KC