Absolutely the 1.5. Not sure what rims you have, there is not a consistent depth or width to the channel, and with the 1.25 you are much closer to being too small for some of the beefier 26" rims (though you'd be fine on others)

The difference between 2.2 knobbies and 1.5 slicks will be palpable. (You'll need new tubes, too) 1.5 to 1.25 wouldn't be too significant and would introduce those negative ride +/- compatibility qualities.