Assuming I'm fighting just one enemy, I'd rather fight the Xenomorph.
-They can be "easily" killed with current-day weaponry. In Aliens the Colonial Marines use M-41A Pulse Rifles which fire 10mm rounds - which exist now (albeit, not in the '10mm caseless explosive-tipped light armor piercing' variety seen in the film). Corporal Hicks is also seen killing a Xenomorph (and possibly more, off-screen) with a 12-gauge Ithaca Model-37 shotgun.
-They're clever, but not particularly smart. In the Special Edition version of Aliens there are a few scenes where the Colonial Marines set up four sentry turrets to defend two possible entrances to Operations/Medical. The Xenomorphs walk right into the turrets' line of fire and many of them get gunned down ('shooting gallery' according to PFC Hudson). After implied heavy losses they let up their attack, even though three of the four guns are out of ammunition and the fourth gun only has five rounds left. In other words, they'll try to brute-force an objective before looking for an alternate solution.
-Assuming I'm fighting just one, it means I'm not going to get cocooned in front of an egg because there aren't any queens or facehuggers around, so even if I lose my death will still likely be swift.
So, to beat a Xenomorph, all I have to do is get myself a gun I feel comfortable using, find myself a hardened room with a single entrance, set up a lawn chair on the opposite side of the room from the entrance, sit down in the lawn chair, point my gun at the entrance, and wait for the Xenomorph to attempt to brute-force its objective. Bam. No more Xenomorph.
The Yautja would probably just use its cloaking device to sneak in, or use indirect weaponry against me, or just brute-force the objective and succeed since Yautja are often seen taking heavy weapons fire like it ain't no thang.