Use case: 2 people, 2 smartphones, need about 700 mins per month, about 200 text messages per person and 3-4GB of shared data per month. We currently pay ~$150 per month for a similar plan on AT&T.
My wife and I both just got out of AT&T contracts, and want to ditch our post paid plans.
There are really only 2 current leaders in prepaid service today.
The first is
Straight Talk's $45 Unlimited plan, which offers unlimited calls, unlimited texts, and "unlimited" data on either AT&T or T-Mobile's towers (choose at time of purchase). I put "unlimited" in quotes because depending on your market, they will cut you off or throttle you for using too much data. Their ToS is written very broadly so that they have the right to throttle at any point, but it really just depends on how many other Straight Talk users are around you. Conventional wisdom is that you get about 2-3GB of data per month. People don't like the idea of not knowing the data cap, but for $45 a month, its tough to complain. Great price.
The second is
T-Mobile's Monthly 4G plans, 3 of which are useful for smartphones. 1) $30 for 100 minutes, unlimited texts and "unlimited" (throttles at 5GB) data. 2) $60 unlimited minutes, unlimited texts and "unlimited" (2GB) data. 3) $70 unlimited minutes, unlimited texts and "unlimited" (5GB) data. T-Mobile is a good choice if you want a bit higher speed and need guaranteed 5GB of data per month (either $30 or $70 plans). The crux of T-Mobile is that the service area is smaller than Straight Talk's and your phone choices are smaller because not many phones supper 1700/2100mhz HSPA+ (the GNex does!).
Do any of the prepaid services offer 4G or LTE data?
None of the prepaid services offer 4G LTE. They do offer HSPA+, which is quite serviceable in the 5-10mbps range, but no LTE.
How does AT&T's prepaid compare to their post paid plans in terms of cost and coverage?
If you're going prepaid, you won't be going for AT&T's official prepaid service (which is way overpriced), like I note above you're best off with another provider that just buys service off of AT&T or T-Mobile.
In general, prepaid has no (or very limited) roaming. In the case of Straight Talk, your service map is about 99% the same as AT&T's official coverage map. If you live in a well populated area you'll have no issues at all.