I thought long and hard about this one. I like both phones. I like the screen image on the Fascinate and the screen size of the DX. Both are too close to the same size to matter much. So in this regard I liked the Fascinate.
However the thing that swung the Fascinate, which I purchased today, came down to the locked boot loader on the DX. I look at it this way. I pay a couple of hundred bucks for the phone and about $1,000 a year for my phone and another $500+ for mifi Internet from Verizon. I think for that kind of money I should have the right to be the superuser on MY phone, not some corporate schmuck (or group thereof). Android is after all "supposed to be" an open platform. Since the root for the Fascinate seems pretty straight forward, I went that way.
So far I like the Fascinate just fine.