I've done a lot of traveling with AT&T, T-Mobile, Sprint and Verizon devices (for work). Verizon was available almost anywhere - by far the best coverage in my observation. Next was Sprint - about 75%. T-Mobile was close - about 60%, and AT&T was in dead last place - about 40%. There was a hotel room in downtown New York City where all, except AT&T, would work fine! So, I stick with Verizon now and have been happy. (I have no financial interest in any of this stuff other than paying through the nose for services)
Get a locked phone and stick with your contract for 2 years - and then you can have it unlocked if needed. But, unless you're a techie, locked phones are just fine.
There are lots of cool apps available for the iPhone. However, almost all of them require payment (from $0.99 to over $35). There are also a lot of cool apps available for the Android phones - and the vast majority are free. Android attracts "open source" developers whose joy is seeing their software used versus Apple developers who want compensation for their work. (my opinion, of course)
Both Apple and Google check software before it goes into their "stores". Downloading Android apps from non-vetted sources can be done and increases your risk factor. There are multiple free virus scanners and malware detectors available for Android. I've run many but never had a detected virus or trojan.
I'll also observe that the world wide share (and the US share) of Android phones is rapidly growing - for ever one iPhone in use there are over 3 Android phones. Samsung is the Android market leader and they make good equipment. The average "life" of any smartphone is 18-24 months because technology is advancing so fast - thus most vendors only provide upgrades for about 2 years.
Having bought many phones (for my family), I'd suggest you head over to your local Verizon store, and check the Samsung Galaxy III. It is less expensive than the Galaxy 4 and works great.