Bethesda. I say this with the acknowledgment that I have never played a Square Enix RPG.
I prefer Bethesda to Bioware mostly because of how combat is done. All real-time (minus pausing to switch weapons) versus real-time turn based (KOTOR). Mass Effect didn't really have that model of combat, yeah, and it's not under consideration for that point. But, mostly, I pick Bethesda because they made Morrowind, which had some of the best immersion and atmosphere I have ever seen in a game. It truly felt otherworldly, where Oblivion--which did have a lot of polishing done to it--felt more like slices of the real world, just tweaked a bit. Morrowind was completely alien/foreign-feeling for the most part. And that's what I fucking loved about it.
Bethesda's RPGs are also a lot more open-ended and provide a lot more freedom/choice in many regards than did KOTOR(II)/Mass Effect, the three Bioware games I played most thoroughly. Never played Dragon Age, so can't talk on that one. In Oblivion, I felt like I had a lot more freedom in how to approach, say, a combat situation. I could charge in there, swords swinging (almost typed "guns blazing"), spells flying, decked out in armor and generally raising all sorts of hell until a layer of bodies coated everything. Or, I could sneak in and assassinate everyone. Or I could summon a minion to do it for me. Or I could waltz in, use my devious charms on everyone, and not have to do any combat. Or better yet, I could pit everyone against each other. I could really approach those situations however I wanted. Granted, a lot of the quests were pretty linear without a lot of choice/branching storylines, but that's true of almost every game nowadays. In KOTOR, I basically had the choice of going in guns blazing or lightsaber swinging while flinging the force in everyone's faces. In Mass Effect, I could go into the same "dungeon" (I rarely notice that sort of thing, but Mass Effect didn't seem to have a lot of variety in places you could go kill things, with a few exceptions for story-important locations) and either shoot everything or magic it all to death. It was still great fun, but less fun in a lot of ways than the crazy shit I could do in Oblivion. Like running in naked and casting "frenzy" on everyone, then invisibility on myself before sitting back and watching all hell break loose in a cave or fort or whatever.
