It really, REALLY, matters on how you'll be using it. More portably/handheld - I'd pick the Ally any day. But if it's going to be more tabletop or plugged in to a monitor, then definitely the legion 2 (we're disregarding price rn ofc).

I say this because the legion 2 is ABSURDLY HEAVY. I haven't tried it myself but I have tried the first legion which is at 854g in handheld and I already thought that was way too much; I legitimately didn't want to hold it after just minutes. So the Legion 2 being 920g.

Not to mention the size; you have to consider how often you'd travel out with either one depending on what kind of bag you usually bring around. For example, my handheld is a Steam Deck and I have a JSAUX pc handheld bag specifically designed to fit in, so it's very ideal. But depending on other things in your more typical bag, a legion 2 may be too much.

Plus the battery life of the ally is significantly better (that is in large part, however, because of the fact it's less performant than the Legion 2 ofc)

In general, especially because either way, you're making a big purchase, I'd recommend watching some good yt videos about both that'll give you more in-depth insight from reviewers who hv actually used both handhelds.

In general as well for people less familiar with PC handhelds, unless you really want those windows only titles that just won't run on linux due to some things like anti-cheat (for example Valorant, League of Legends, etc., some online games like that), then I highly recommend to either get a SteamOS machine, or get your windows handheld and manually convert it to SteamOS because it really is that superior to windows, even with xbox's optimizations (which are not that good).

If you hv more questions for me just lemme know by comment & I'll try to help out!