The Vibrant and MyTouch Slide are both excellent phones, I've tried each for a week and decided to stick with the SLIDE.
The Vibrant is a better multimedia device, so if your focus is watching videos, TV, gaming and awesome surround sound options for music, then go with the Vibrant. The Vibrant is also very eye catching and super cool to look at when you whip it out of your pocket. The sensor rotates on all sides for orientation, that may make a difference with gaming.
The MyTouch Side has a more durable feel and may take more of a bruising, if you are hard on phones. I also felt the in call volume to be better on the Slide than the Vibrant, especially in public spaces like a coffee shop, train station or soccer game. If you use a bluetooth religiously, then the in call quality on the Vibrant is greatly improved. The music is very nice on the MyTouch, but it was AWESOME on the Vibrant. Then there is the physical keyboard on the MyTouch, if you can't get into the SWYPE.
As for speed, the processor is faster on the Vibrant, but I can't tell a significant speed difference for downloading mail, accessing the web or downloading music. I think because HTC has been doing Android longer, they are more effecient in their design and can do more with less.
I also prefer some tactile sense on the MyTouch Slide. I can make a call using the GENIUS button withouth looking at the phone. This is a help when driving. ie.... Upper left corner press down with index, swipe down with thumb, press right corner button with thumb, wait for beep and speak your command. I can't do that with an all touch screen without looking.
Good luck with your final decision and please let us know which one you selected.