I've actually been pondering this very same question. It sounds like there's a design flaw with the Ally though - Basically, it will cook your SD card, which will, of course, cause it to no longer work. This is a hardware issue, so there's no fix for it, short of a recall and redesign, which ain't happening at this point. The regular Ally (Z1) is only $399 at Best Buy, which is tempting to me, since I have never used an SD card in my Steam Deck anyway.
The Odin 2 seems cool, but it sounds like the hardware has weird input lag and Ayn isn't interested in addressing it. This is a deal breaker for me, so I've decided to pass on the Odin 2.
So yeah, I guess I would go with the Ally if I had to pick one, but I think I'm going to sit this round out.
Edit: All that said, I got
this Beelink mini PC near the end of last year and I discovered yesterday that it can run PS2 at something absurd like 4x native res, which is simply incredible to me. I'm having a lot more fun messing with that than any handheld for right now.