I cannot think of any prepaid plans in the U.S. that meet all of your requirements, though I am far from being familiar with all of them.
Here is what I (think I) "know", though I would love to be set straight on anything I got wrong or missed:
Generally 3G is available wherever LTE is. Between the national GSM/UMTS carriers, AT&T generally has better coverage than T-Mobile does outside of metropolitan areas, so if you are traveling a lot between metros in something other than an aircraft then this is something to consider (though it doesn't sound like this would likely be an issue for you). In some areas, AT&T and T-Mobile might allow you to roam between their networks, but I believe this is pretty rare (esp. on prepaid), so you should plan on being forced to make a decision to be on one network or the other, and just be satisfied with the native coverage of whatever network you pick.
The larger issue for you with T-Mobile might be their previous (and ongoing?) use of AWS bands (split 1700MHz uplink/2100 downlink) for 3G service, which very few phones sold outside of the U.S. support. They have been working nationwide to reallocate their AWS licenses to LTE, turn off 2G service on PCS (1900MHz) bands, and move the 3G service to PCS, but my understanding is that it has been taking them a while to complete this migration and I do not know the status of it in any given market.
If you are planning to bring your own phone, then don't bother with Verizon or Sprint...not only are they generally hostile to "bring your own phone", but their 3G coverage consists of SIMless EVDO with a fallback to CDMA/1xRTT for voice, NOT GSM/UMTS. Your phone most certainly will not support this.
Very very few (if any) prepaid plans, either from the carriers themselves or MVNOs, have roaming in Canada (data or otherwise) built-in to the "plan". You will likely have to buy a separate SIM+plan while you are up there. I have zero knowledge about what the Canadian wireless market looks like so cannot help you there unfortunately.
I can also think of exactly zero prepaid offering by either AT&T or T-Mobile that give you non-NAT IPv4 access. As others have said, T-Mobile does offer native v6 (dual-stacked with NATted v4), which of course would not be NAT. If you need publicly-reachable IPv4 you might consider VPNing to an access concentrator that will hand you such an IP when you need it...I happen to know that L2TP+IPsec w/ NAT Traversal happens to work over v4 on most AT&T prepaid plans and the plans of their MVNOs.
One popular MVNO here is Straight Talk, a sub-brand of America Movil. They will only sell in blocks of 30 days minimum, but you can get 8GB over 30 days for USD $45 or 12GB for $55. If you exhaust your data allocation then it doesn't cut you off, just throttles you down massively. But if you do exhaust and want full-speed back, you can choose to waive whatever remaining days you have left of the 30 you initially bought and immediately refill your SIM with another 8 or 12GB purchase, which starts the 30 day clock over. The only way to get a SIM in person instead of by mail-order is to buy one of their activation kits at retail from a Wal-Mart store...I believe they are USD $60 and it includes SIMs both for the AT&T and T-Mobile networks (you must choose only one at time of activation) plus a $45 8GB service card (so the SIMs themselves really only cost $15). Unfortunately, if you want the 12GB service plan, other than ordering just a SIM from straighttalk.com and having it shipped to a friend in the States ahead of your travels, I don't know of any way to accomplish that while also being able to avoid paying for the 8GB service card that is included with the retail kit.
If you shop for other MVNOs, be very careful to get clarification on what carrier's network they use before you fork over any cash. Many of them only offer access to one carrier, and if that carrier happens to be Sprint or Verizon then no way will it work for you (and there are an awful lot of Sprint-only MVNOs in particular). Just because a certain vendor offers a SIM card doesn't mean it is for a compatible network...Sprint and Verizon use SIM cards for access to their LTE networks.
One (admittedly more fiddly) option you may want to consider, which would allow you to use whatever carrier you wanted without needing to be concerned about the underlying network technology or support for particular bands, would be to look into either purchasing a cheap "MiFi"-like device, or see if one of the providers will rent one to you for the duration of your visit. Then you could jump on, say, Verizon's LTE network with the MiFi and have your phone talk to the MiFi with WiFi. You said you were likely to use VoIP for voice communication anyway so not having a SIM in your phone doesn't sound like it would be a problem. (This may not solve your Canada problem, though...you'd still likely have to work out a separate solution for any time spent up there.)
Hope this helps,