I was between these two as well, and went with the T61. The main reason for me was lack of an optical drive on the X60. I know there will be at least one time when I want to use one while out and about and will be annoyed. If i were to carry around an external optical drive, or the expansion station, it would totally defeat the purpose of the small machine.
Plus, you can get more power for less money with the T61. I actually think of everything out there on the market, the T61 has the best size/weight to power trade off for the money (with the discounts they are running). At the end of the day, its just a little bigger and only 2 lbs heavier, but it has a bigger screen, more power and is less expensive.