I have owned a few HTC Android devices (G1, MyTouch 3G, Nexus One). I love HTC devices. I have never seen a Samsung Android device for which I would give up my Nexus One (but I wouldn't give it up for anything, anyway).
Samsung has been known to lag behind in Android updates. My other issues...
No, you won't get billed. You can't be charged for using a network that the carrier doesn't own, right?
You don't pay any extra on your Internet bill for using a cell phone with wi-fi.
I don't know what you mean by the last question. Free means you don't pay for the app.
If there hasn't been an upgrade from Sony or your carrier (you would have received a notification), you can probably root the phone and install a ROM of the newer version.
Start here:
http://forum.xda-developers.com/forumdisplay.php?f=723
Do you have a contract with T-Mobile? Then you have to pay an early termination fee. You agreed to this when you agreed to the contract. Unless you pay that fee, you're stuck with them until the contract is up.
The registration you did at the initial setup was for your Google account syncing. All Android phones have to do that. That has nothing to do with Sense. Any Android phone can upload images to Facebook, with or without Sense.
The cost of a data plan has nothing to do with the phone. They nearly always cost the same. I would get the MyTouch because it's Android and can use T-Mo's 4G network speeds.
You cannot buy an iPhone for $199 without a contract from a carrier. The retail price of the iPhone is about US$650. The lower prices are only for phones on a contract with a carrier.
Since the USMC is part of the US Navy, there is a chance you will have some kind of sea duty. You may be assigned as a security detail on a nuke carrier, as an example. Most Marines probably don't see any ship time, but if that's where you get orders, that's where your going. They will help you...
Let me keep this simple: buying an iPhone, trying to unlock it, then using it on T-Mobile's network is a monumental waste of money.
Think about what you're asking: T-Mobile has already deployed they're HSDPA+ network (3G+) and is rapidly deploying their 4G LTE network. They have really great...
Yes. Great phone made by a great company (HTC), fast network response with T-Mobile (if you have 4G service in your area). If I didn't love my Nexus One, I'd buy one.