My devices are both T-Mobile devices that I unlocked. So they have the hardware for T-Mobile 3G/4G, but only 2G with AT&T. I've tried both a Straight Talk T-Mobile SIM and a Staight Talk AT&T SIM just for testing's sake.
The Straight Talk T-Mobile coverage and speeds are exactly what I was seeing when I was a T-Mobile customer. With AT&T, I get coverage in more places, but only at 2G. Where I live has AT&T coverage in more places, so this was the expected outcome. I use T-Mobile for regular use and get a spare SIM for both networks. So when I travel to an area with no T-Mobile coverage for days I can switch over to AT&T's network temporarily and then back to T-Mobile once I'm back home.
Once you stop using a SIM, you cannot reuse it again for your plan. So once you switch from say Straight Talk T-Mobile, to get back on T-Mobile you need a new T-Mobile SIM and vice versa for AT&T SIMs.