Touch screens vary quite a bit between phones (capacitive vs. resistive, and the different sensitivities between screens). I personally like very sensitive resistive screens best (like on the Nokia N900), because they're as easy to use as a capacitive screen with a finger, but also work with long finger nails and gloves when its freezing outside.
Capacitive screens won't work well if you have long finger nails or want to click/select very small things (like links on a web page without having to zoom way in). However, they are usually very sensitive, which is good. The worst kind of touch screens are screens which are not very sensitive, because you have to press very hard on them for them to detect your finder, making them difficult to use. I've used at least one LG phone (can't remember the model) which had this problem.
I'd suggest you go to a store and try screen on the phone you want, and see if its sensitive enough. Smudging really isn't a very big problem, and if you think scratching will be a problem, a cheap screen protector will take care of that.
QWERTY is much better than a flip phone. On-screen keyboards also suck, so make sure you get a phone with a physical keyboard, even if it also has a touch screen.