Verizon and AT&T are pretty much the same in terms of pricing. There may be small differences in features or subsidies for new phones, etc., but by and large, they're the same.
The big difference is in their hardware offerings and the behavior or their network. Assuming you can make your own hardware decisions, I will sum up network differences like this: AT&T's "3G" or "4G" (depending on who you ask) is a GSM based HSPA/HSPA+, and it is very fast. The caveat is that it is not always available, and the highly populated areas create a lot of congestion on the network.
Verizon's EV-DO Rev 0/Rev A, "3G" is slower on average, but has more consistent performance. If you opt to get one of the LTE "4G" devices, it's blisteringly fast...super-greased-lightning-fast...faster than your computer could process if you tether/hot-spot...
The only "4G" devices right now are either Android devices or MiFi/USB devices. Personally, I would keep myself open to all the platforms if you're going to make the switch. Examine your needs and decide from there. The Bold 9650, Storm2 9550 are both Global capable. The Droid Pro, Droid Incredible 2 and Droid 2 Global are as well. The new HTC Trophy (WP7) is too. There are a few older legacy type devices as well.
If you are concerned about international roaming, unfortunately, the unlimited international data feature was discontinued earlier this year. Despite that, believe it or not, as stupid as the pricing is now, it's still noticeably cheaper than AT&T.
Share what kind of goodies you end up getting.
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Verizon or ATT?! 
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