As they say, the best bike for you is the one you actually ride -- and the more you enjoy it, the more you will ride it!

I had the opposite experience; I commute regularly on a Surly Crosscheck; it was out of commission for a few days while I was working through some crankset issues, so I rode my MTB (GT Backwoods, front-suspension only), and HATED it. Not only is the seat an ass-hatchet, the knobby tires, UGH, the commute was such a drag! When I got my XX back on the road, it felt so good and smooth and fast!

All that to say, I would recommend giving a try to some slick or at least "city" tires on your Schwinn, then you can roll smooth and fast AND in plush grandma's-cadillac comfort. Putting these Nashbar Slicks on a MTB is what launched me back into being a cyclist again, after 15 years of not riding. They're very well reviewed, and dirt cheap (only $12 each, and a free shipping deal right now) so not much to lose. Nowadays I put these (and a rack) back on my MTB when I want to tour -- because my engine needs MTB gearing to get up SoCal mountains!