Can any tell me how good are outdoor heat and air conditioning packages are.?

I'm thinking of getting an outside heating and air conditioning package for my home. I was wondering how well they work and how energy efficient they are. Also where is a good place to buy a unit. I need a 2.5 or 3 ton unit. I might stay with propane heat since I have an existing propane furnace now. But I might go with all electric if it is cost efficient.