As someone who really enjoys Metroidvanias/2D “single area exploration” games, I’d have to give the preference to Axiom Verge (I played it on PC and Vita, and the Mummy on Switch, so if there are issues with Axiom on Switch, I’m not aware). I had a number of issues with Mummy (slowdown in the later areas dropping framerate to almost single digits, and the final boss’s AI stopping altogether midfight, making for an anticlimactic end - but I believe those issues have been patched since release)
Both are very good and interesting in their own right, but I find Mummy to be a lot more derivative, both aesthetically and mechanically (no power up is something we haven’t seen before, and most additional movement options are essentially lock and key design, not tools that you can creatively use to better traverse the game’s space). Since you’re not familiar with the genre, however, that might be a positive for you, and as Gustaf said above, an interesting part of Axiom Verge is how it subverts expectations relative to the genre.
With that said, I find Mummy also a lot harder for the wrong reasons (enemies hit very hard, initial damage knockback is very reminiscent of the earliest Castlevania games but the level design isn’t built around that kind of knockback, making you fall without a way to climb back up far too often), and the overall level design doesn’t really encourage exploration (you’re guided by narration given signposts, rather than by organic clues in the levels themselves).
I believe Axiom is a tighter and more mechanically satisfying experience that showcases the strengths of the genre well (subvertion or no), but Mummy wouldn’t be a wrong choice either, and if at all possible, I’d ultimately recommend both! (additionally, keep a look out for Hollow Knight, still supposed to release on Switch eventually - as far as Metroidvania goes, I feel it’s one of the better modern takes on the genre!)
As a side note, I also very much enjoyed Dragon’s Trap, but I’d recommend any of the other two for a “metroidvania” experience, as Dragon’s Trap has a slightly different design going for it. But it’s an excellent game in its own right (though be sure to adjust your expectations and sensibilities for a late 80’s platformer ), and well worth your time, too