iPad 2 is equivalent in CPU/RAM to the original iPad Mini. You definitely want to avoid that one.In general, the newest one with the fastest processor would be best. Nor sure about the iPad Minis, but I had no end of trouble in live performance as well as significant latency using Jamup/Bias with an iPad 2. It would work OK for testing, but in live performance went down on me several times right before the set started.
Interfaces that connect through the charging port have the best sound but the highest latency. I have read the latency with a digital interface and newer iPads is low enough to be insignificant to some players. As a comparison I measured the latency with my older iPad 2 and a digital interface (using Bias) to be around 40mS or so, which is unusable IMHO. So newer/faster iPads are even more important if you are using a digital interface.
I did not like the L6 Sonic Port because it didn't respond well to rolling back the volume and/or tone on my guitars. My Apogee Jam 96k responds much better. My Roland Duo-Capture EX also works well with the iPad if I need a more full-featured interface.Is the Line 6 Sonic Port still a decent interface?