Well, if you have an unlocked GSM phone or a tablet, you can order a $1 SIM from T-Mobile and try it out. T-Mobile is pretty easy in that regard. As soon as I popped in the SIM it picked up the network, and from there I used the phone/tablet's stock brower (Chrome) to buy post-paid plans. It's similar to buying daily Wi-Fi time at hotels. I dont' think there is coverage area difference between T-Mobile's pre-paid and post-paid. At least that's what I heard from a couple of reps over the phone.
Only way to "try out" AT&T LTE network that I can think of is to buy their post-paid LTE phone. You can buy them from Amazon or Bestbuy but they are not cheap. And for AT&T, I was told there is a coverage difference between post-paid and pre-paid.
Or perhaps you can try one of these? AT&T stores don't give out SIM cards no matter how many times I threatened them. lol.
http://www.amazon.com/AT-Wireless-Ne.../dp/B004VHI708
Edit: If you decide to go with SIM tryout route (AT&T or T-Mobile), make sure to set the correct APN. (access point name) Google search usually yields right APN settings for your phone.
I tried out T-Mo prepaid, and my data speeds were horrible. I had probably 2-3 bars with 4 being reached every so often. Also, it would get a 4G connection (at least that's what I could tell on the Lumia 521). When I ran a speed test regardless if it were 2G or 4G, the ping was in the thousands and it couldn't finish a test meaning it just sat there waiting to download and upload for about 30 seconds a piece till it timed out.
I would like to try AT&T, but the more I think about price, the more I want to be able to use T-Mobile since it's prices are ridiculously cheaper in comparison.
This is what I get on T-Mobile Pre-paid in urban Houston (2x5Mhz LTE):
By the end of 2013, 90% of their LTE footprint will be 2x10Mhz (so doubt the speed). And I should note that even though I got LTE when T-Mobile launched it in March, I've only just recently noticed a HUGE bump in tower signal to get these speeds everywhere. Before it was just tiny pockets near a tower. So it's just your area and whether they've worked on it or not.
I don't know what is the matter with my area. Clearly, I'm supposed to have pretty solid signal (I know coverage maps lie), but when using the 521, it just felt like since I was prepaid I didn't get the service I was supposed to. I know I can get 4G with AT&T. I have a VZW iPhone 5 and used my sister's nano SIM from AT&T, and it worked flawlessly. I pulled speeds of around 7-8 MB/s so I'm not worried about trying out AT&T anymore... I should probably change the thread title/OP.
You may want to try out the Lumia 520, a low end Windows 8 phone. I saw an article that Amazon was selling it for $80. The 520 is a 4G phone so this would be an inexpensive way of trying out prepaid ATT and see if you get better service. The 520 comes with the Amber and GDR2 upgrades already loaded on it.
Don't worry about AT&T devices since the iPhone 5 from VZW can use AT&T spectrum (not LTE though which my area has yet to get anyway).
To be honest with you, I don't make much money which is why all of this is such a struggle for me. Also, it just sucks that one GSM network apparently doesn't have good signal where I live, and the other is just extremely expensive for what I want.
*le sigh*