So here's the deal....
You have four carriers to choose from (don't even think about MetroPCS and that shit), and two categories that they fall into. Sprint and T-Mobile fall into the "Cheap" Category and Verizon and AT&T fall into the "Expensive" Category. Why would you even think of going for the expensive ones? Because you get what you pay for.
Sprint and T-Mobile offer slower speeds, less coverage, worse in-store customer service, less selection, and less versatility in the plan choices. That's why they are cheaper. It's like anything you buy in life. Audiopipe or Sundown. Curve or Armani. Kia or BMW. Whatever your budget allows should be the carrier you go for, depending on your tastes.
There is one exclusion here. Some carriers just work better than other in different areas. In my area (Vancouver, WA), Verizon and AT&T are the obvious choice if you want the best coverage. In Nateberrier's area (who cares, Ohio), Sprint is the obvious choice for the best coverage. It is not always the case the Verizon and AT&T will have the best coverage where you live. HOWEVER, it is the case that there is a greater chance that they will do better in your area solely because they have a larger network. Just ask around town, find people who have used multiple carriers and don't hold a bias and see whom they believe has the best reception.
From here on, I'm just going to assume you're going the AT&T/Verizon route. Let me preface by telling you that I work for AT&T, but I am not in any way biased towards them. I LOVE my job, but I don't let that cloud my judgement. In the end, you'll see that I would choose Verizon for my phone service if I had the choice.
Here's why you SHOULD choose AT&T: Wide phone selection, fantastic in-store customer service, phones are generally much cheaper than Verizon, 5 different levels of internet that are all backwards compatible (GPRS, Edge/2G, HSPA/3G, HSPA+/3.5G, and now LTE/4G).
Here's why you SHOULDN'T choose AT&T: Dropped calls are prevalent in any device (especially the iPhone) and are frequent, HSPA+ is not noticeably faster than HSPA, LTE only covers 14 cities if I remember correctly
Here's why you SHOULD choose Verizon: Expansive LTE network, extremely stable network (no dropped calls), at the moment they are offering double the data for the same price (4gb for the price of 2gb)
Here's why you SHOULDN'T choose Verizon: Not quite the selection of phones you get from AT&T, big speed gap between LTE and 3G, not the best in-store customer service
Before LTE, there was only one thing that truly separated Verizon from AT&T, and that was dropped calls. Today, Verizon now has the upper hand on both, and it's those reasons why I would choose Verizon. Just keep in mind that Verizon at the moment prices their LTE phones much too high, but they are still worth it because LTE is so goddamn fast. It really is a huge difference.
Phone choice is a whole different situation. We have 4 real contenders in the smartphone market for software, those which would be iOS, Android, Blackberry, and Windows. Here's what it really comes down to...
Simplicity, great for new users, huge app store: iOS
Little bit of a learning curve, but lots of things to customize, huge app market: Android
Lots of business functions: Blackberry
Same reason you'd go for iOS but a different flavor and less apps: Windows
So if you choose to go for Android, it's because you're not scared of learning how to use a phone and you want to get the most out of your smartphone experience. I really wouldn't suggest Blackberry or Windows phone unless you want it only as a business phone, or you want a simple to use phone that isn't iOS.
Now that you have chosen your favorite software, it's on to phone selection. I'm going to focus on Android because there's no point in delving into iPhone selection when there's only a couple to choose from lol...
We have a bunch of different companies that manufacture Android phones, but the main players are HTC, Samsung, and Motorola. Here's what you usually get from each:
HTC: HTC's Sense overlay is arguably the best of the 3, their phones typically have the best build quality and great on-paper specs. Unfortunately, they never usually perform as well as their on-paper specs would suggest, with good build quality comes bulk and weight, and battery life is almost certainly very poor on an HTC phone.
Motorola: Usually the leader in innovation and features, phones are pretty fast, typically priced well, and almost always the best battery life of the three. But, MotoBlur is obviously the worst overlay of the three, and the phones are usually the least reliable.
Samsung: Known for making some of the thinnest and lightest phones, usually the phones are unbelievably fast, Samsung's TouchWiz overlay is decent, battery life is usually decent or better. The downfalls are the poor build quality (cheap materials)
I usually never suggest Motorola phones because they are almost all terribly unreliable. Note that I said "ALMOST all". There are some exceptions. But for that reason, I'll usually go for an HTC or Samsung phone. When it comes down to it, Samsung is my real favorite because of their lack of compromises.
For AT&T, the obvious choice right now is the Galaxy S II Skyrocket. For Verizon, you should wait for the Galaxy Nexus or the upcoming Quad-core HTC phone if it comes out.
Hope this helps! It's a lot to read for sure, but it should aid you in your carrier/software/phone selection.