I may be wrong but I believe that if you have any issues with your phone and you bought it from someone other than Verizon, then Verizon will not make any repairs or provide you with in-store support. That may not be a $150 difference to you, but just FYI.
I upgraded to 2 Samsung Focus', one for the wife and one for me, earlier this year using Amazon. It was amazing to me to see the $0.01 charges on my credit card. There are no catches. Just be sure you can live with the early termination penalties - which don't appear to be unreasonable compared to others. My experience was excellent. I'll never go to a store and upgrade in person ever again. It's that painless. I'd upgrade using Amazon again definitely!