But I am under the impression that all iPhone models now have the same innards. Not true?
All of the iphone 4S's had identical innards. They had both the GSM and CDMA radios. But Apple has abandoned that approach with the iphone 5, probably because they have had to add the LTE radio. So if you kept your iphone 4S and got it unlocked, you could use it both in the US and while abroad. But not so if you upgrade to the iphone 5/c/s. There are also a lot of Verizon Android phones that work using all three technologies CDMA, GSM, and LTE....most of the Motorola Droid phones over the past couple of years, for example. Once unlocked (which Verizon will do for you) you can use it pretty much anywhere in the world. I wouldn't recommend using either Verizon's or AT&T's international plans, though. They are much more expensive than purchasing a local sim card when you travel to a new country. Using a local sim here in Indonesia, I pay a few cents/minute to call the US, while on AT&T's plan it would cost a couple dollars/minute. And for that matter, if you don't need LTE, you can use either the Verizon or AT&T domestic networks for about half price using StraightTalk, PagePlus, or one of the other MVNO's.
[Edit: You can
get AT&T's LTE service through StraightTalk now...unlimited minutes and data for $45/mo. No affiliation...]