X-1: Long combos means it's basically a sitting duck for most suits. It's unique skill doesn't do a whole lot for the hp activation required. Auto potential is nice, but the boost part is just regen not max boost gauge amount & anti-beam is barely noticeable.
With the buff, the ranged mode is actually viable, however other rocks still outclass it. If you're willing to wait, I've been hearing X-2 is more efficient than X-1 is.
F91: Dated suit, which isn't used in mission grinds as much post nerfs. However, it's still potent in stock, and it's alt mode gives it multiple mobility options. In addition bio computer is a pretty amazing skill.
Generally speaking F91 is superior overall, and if you have all the mats already I'd go ahead and mix it. Though I'd look first at what you have for overall suits, and decide from there. If you are lacking in papers plan F91, likewise if you are lacking in rocks. If you have a lack of both, I'd give the nod to F91 just because of it's versatility.