I like the samsung vibrant better to me it's nearly perfect as a android phone. some things bother me though:

the back panel looks too plasticy/cheap (captivate back panel better)
i've been seeing huge reception drops (though no dropped calls) for both the captivate and vibrant on youtube:
http://www.youtube.com/watch?v=dogM4Wu51tc
http://www.youtube.com/watch?v=jLJPMcPUi7E

But on the brightside it's still a great phone
no dropped bars even with 0 bars
has the humming bird 1gh cpu (similar to the one samsung manufactures for iphone)
great graphics performance
a really polished OS with less problems than the captivate

I believe the captivate and vibrant will erode the consumer base of the iphone, =P and rightfully so. It's everything the iphone is and better, more polished os, simple UI, bigger screen, and with more features such as flash video