3' should make little difference as far as towing goes. Some things it will effect are;
1) Weight. Does the extra weight push the limits of your tow vehicle?
2) Fuel Costs. Will the probable extra weight add to your fuel costs and will that bother you?
3) Comfort. Will the added length (and probable extra weight) give you "creature comforts" that you want?
4) Parking. Will the added length cause any issues when parked at home or in the parks and sites you normally visit?
Two years ago we traded our KZ (that we loved but was beginning to its age) for our present rig. We went down 21/2' and have never really noticed the difference. Actually, the layout we have now actually works better and feels larger most of the time.
Weigh your options carefully and make the best decision you can. Do that and you'll probably be happy.
Happy Camping!