.... the best choice for you would be the myTouch 3G Slide
God no, if you are coming from the power of a N900 and want Sprint so you can be on a family plan, Epic 4G about to come out has the keyboard you are looking for. If you want Verizon then there is the Droid 2 about to come out. T-Mobile has the POS myTouch 3G slide (keyboard sucks on that thing imo). ATT nothing with a keyboard.
Sorry for the Offtopic, but I really don't get that US system of buying phones. What shitty system is that where you have to buy a phone based on whether your service provider sells it...?
Over here you MIGHT buy the phone from your carrier, but only because it's "free/cheaper" when you sign that 24 month contract (with monthly fees higher than normally, so actually you end up paying more for the phone), but the normal way is to just simply get the phone from the thousands of alternate source and freaking plug the SIM card in without caring for your carrier's feelings about your new phone... :P