it's a hard question since wireless range depends on many factors. the materials of your house, drywall, brick, other neighbors networks, if the station will be in the corner of the house, if you have a microwave or cordless phones, how may computers are on your network, the list goes on.
I have found that all things equal they both are about the same for Wireless G networks. so if you are planning a G network, the extreme gains you nothing but it does future proof. the express is only G enabled and the extreme is N enabled. If your computer is only G then they will both be the same. if you have a N computer you will see a much better through put with the extreme (faster file transfer) and greater coverage.
So assuming you are content with G I'd go with an express. i have a powerbook G4 that can pick up my express network at my neighbors across the street or in my basement from the second floor.