My vote is the Ti. It comes down to a tradeoff. For the Ti you are giving up an additional exchangeable bit driver and gaining a pair of scissors. If you don't ever use scissors, and don't plan to, maybe the XTi would be your best bet. I have never had a use for scissors, until I decided to try them. I would hate to be without them now. They work great for delicate operations.
Keep in mind that once the extended bit set comes out next month you will have many differents bits to choose from. As long as you have the Ti in a carrying case, you won't ever lack for a bit you need since the case will carry about 10 extra bits. However, if you carry the tool by itself as a pocket knife, then you would lack that extra bit.