Today only the Vibrant is actually in T-Mobile... Wednesday the myTouch 4G will out... and who knows when or for that matter if the Nexus 2 will be out or even available at T-Mobile.
So, providing you are willing to wait until Wednesday... both the Vibrant and myTouch 4G have screen keyboards, the Vibrant has a Super AMOLED screen, the myTouch 4G a Super LCD... screen size, and processor speed are the two other differences even though they both have 1Ghz processors the myTouch 4G has the next generation processor.
I prefer the size and feel of the myTouch 4G over the Vibrant... I got to handle one over the weekend at one of the stores. I can do without the larger screen size.