I really liked having a keyboard on the Slide and it had some nice Slide specific apps. The main reason I got the Vibrant is because it has more space to hold my music, 48gb instead of the Slide's 32gb. The two selling points of the Vibrant are the big screen and the fast processor. The big screen doesn't matter to me, cause if I'm going to watch a movie I'll just watch it on my computer. But the processor is almost twice as fast as the Slide's and I'm sure I'll appreciate it. I can't say I've noticed it being faster than the Slide though, but I haven't tried it out very much yet.
My pros of the Slide: Slide out keyboard, Nice apps, liked the look and feel a lot
My pros of Vibrant: Big screen, availability for more space, really thin
cons: only touch screen, side buttons are only volume and power
These were my first android phones and I've only had them for a few days so far. Hope that helps.