Call me an idiot, but I hadn't even considered the possibility of using WiFi. So when I'm at my house I can just connect to my WiFi connection and have great speed in my phone? That's great, because my house is pretty much the only place in the T-Mobile coverage map that doesn't look very good. I work in Houston, where there's great coverage, so my house is the area I would be the most worried.
Yes, using WiFi at your house has the advantage of giving you WiFi speeds as well as not using your data. As was pointed out, text and data would still be dependent on cell service but you don't need a good data connection for them to work well.
I had Virgin Mobile (Sprint) prior to switching to the $30 T-Mobile plan and am in the North Houston area. I tended, more often than not, to get 2G speeds on Sprint even if I had a strong 3G signal. In fact, there were lots of times I had speeds under 100 kbps. While Sprint has a solid signal, there have been too many people using Sprint's towers causing data speeds to die. This has improved a little, since July when LTE came on for Sprint in Houston, but 3G speeds still are slow. And Sprint pre-paid, through Boost or Virgin Mobile, cannot access LTE but use WiMax (for the few phones that have 4G) -- though WiMax can have a lot of dead spots. My T-Mobile HSPA+ speeds have been as good, or better, than WiMax with better availability.
About what you mentioned of straight talk, it actually looks like a great choice. I tried to look at the Straight Talk coverage map but you can barely see anything, so I looked at the AT&T coverage map in their website and it's absolutely great in my area, getting in fact 4G all over the place. Now, does this mean that Straight Talk will also get exactly that? I am not sure how that really works. I also like the fact that I would get unlimited minutes and not have to worry about going over the 100 minutes from the T-Mobile plan, but of course you're also paying $15 more a month.
My understanding is that the Straight Talk coverage should be roughly equivalent to AT&T's, with the exception that it doesn't have LTE. There are some complaints about Straight Talk being quick to throttle users (after 2GB/month or even 200MB/day). My understanding is their terms of service says that users are not allowed to stream video or music -- though asking that question on the Straight Talk forum would likely give better answers.
In my case, I use Google Voice combined with GrooveIP when connected to WiFi, so that I'm using VoIP rather than my voice minutes, to extend the 100 minutes. I spend most of my day connected to WiFi, so it has done well for me so far.
One more question from a phone newbie. As you say, I wouldn't be trapped in a contract, so I could get the T-Mobile plan and if it doesn't work great I could change to the Straight Talk. But how does that work regarding keeping your phone number and that kind of thing? Let's be specific: I am using a Tracfone right now, and want to transfer my number to my future Galaxy Nexus. I thought the way to do this was getting a SIM card from T-Mobile, and then doing this. What happens if I decide later to go with Straight Talk? Will I have to get a Straight Talk SIM card and do exactly the same? Any I will have no problem keeping my phone number all the way?
Yes, if you try T-Mobile and decide it isn't working for you, you just order a SIM card from Straight Talk and use it to move your phone to their network. You should have no problem porting your number from Tracphone to T-Mobile and, if needed, then over to Straight Talk. When you activate the SIM card, you merely let them know that you want to port your old number.