I have the merge and really like it. If you are interested in the side slide it is very nice. Don't let the middle of the road (by today's standards) 800mhz processor worry you. I have never felt this phone to be laggy in the least. And now with the update to Sense 2.1 and Gingerbread it really is a complete phone. It is a little heavy but feels very well built. Screen is crisp, call quality is good. My only complaint is sort of twofold. Without a case it is somewhat difficult to slide open with the tight spring mechanism and the smooth rounded edge around the screen half of the phone. And the second half of the complaint is that the USC case that snaps on to the phone, while making it easier to open and close, sometimes causes the back battery cover to come of easily. I realize it isn't easy to get a case to stay on a slider compared to a candybar style phone but still...
All that said, I am going to look at the Motorola Electrify (which is the exact same as the Photon thru Sprint) very seriously. My two main reasons are the brighter screen as I work outdoors a lot, and the ability to get a good solid otterbox case for it as I'm a little rough phones. It should be out in the next 30 days.
So I don't think you can go wrong either way. If you need a phisical QWERTY then the Merge wont dissapoint. If you want a little bigger screen and some added features like front facing camera, and faster internals, the Electrify will be a great choice.
I'm sure the Electrify will be priced a little higher too.
From what I've seen these will still be your two best choices till the end of the year. The only phone I wouldn't be surprised to come to USC early next year would be the Samsung galaxy s 2. But from all I've seen it really isn't significantly better than the Electrify nor is it a sure thing to come to USC.
Hope this helps!