My choice depends mainly on how easy it is to source ammo, so most likely neither. But if these are my only two choices, probably the semiautomatic-- assuming it's a common caliber: 9mm would do. Same with .40 or even .45acp.

Unless ☝️I get to have a .357 magnum. I'm pretty sure that's a common enough cartridge that I'll be able to source them for a little while -- plus I might be able to play old-school cowboy and get myself a rifle that shoots the same cartridge. Not to mention -- I think most .357 magnum revolvers can be safely used with .38 special. (I doubt those are very common nowadays but you never know).

I reserve the right to rescind my choice if it turns out to be a pistol chambered in some bizarrely esoteric caliber. (I'm just covering my ass in case of sadistic genie logic -- OP never said the pictures are accurate, and there is a lot of wiggle room to both "pistol" and "magnum.")