Tough, tough choice.
In the end I have to go with Bethesda. Bioware releases a tighter product, with better writing on average, but their games also tend to be a lot less playable. To more casual players (and a lot of people who are casual don't like to think of themselves that way) having a massive sandbox RPG can be intimidating, and Bioware RPGs are a lot more streamlined with a plot that pretty much ferries you along to your destination, with say a choice of which path/plotline to move down at any given time, with the metaplot concluding when all of the lesser paths are completed (ie find the Jedi hiding on three differant planets, you can do them in any order or move around, but still follow each linear progression. Once all three are done the overall plot moves on/concludes). A plot you can ignore as long as you want and the majority of the game material taking place outside of the central narrative means that there isn't a conveinent set of guide rails for people to follow to see all the "good stuff" and that's just too intimidating for a lot of people.
Understand I have fond memories of games like the original "Might And Magic" which were like adventuring rather than a continueing Drama. You started out with a team of adventurers wanting to do cool things, and the nebulous objective that discovering the secret of the Inner Sanctum would be the most impressive thing someone could do in that world. You'd run around, explore, defeat lesser enemies and minor plotlines like a Succubus Queen enslaving a town and so on, while finding pieces of codes and messages that when puzzled together would lead you down the path to figuring out where The Inner Sanctum was, and how to access it. Especially for the time, the secret was also a good one, even if hardly original even then (if you read a lot of fantasy/science fiction novels that is).
While I'm waxing nostolgic, I will also say that I think when people sit down and start prattling off lists of the greatest computer game villains of all time, I find it disturbing that "Sheltem" who was quite cleverly and subtly introduced in the beginning, and a rising threat through a number of Might and Magic games, is frequently overlooked. Especially seeing as Sheltem actually had a better backround than a lot of the villains out there, not to mention having an arch enemy (and the party's patron) who he had already killed and was literally operating as a soul in a box. While the graphics are dated the final duel between Corak and Sheltem was a big deal in it's day.
http://www.youtube.com/watch?v=Zg4q8v6ku5o
At any rate, despite all the nostolgia the bottom line is that while it's ideal to have both an awesome story and a sandbox, I'll take freedom and depth over storyline almost any day if I have a choice. Especially seeing as by the third or so time I've heard dialogue I wind up fast forwarding through it, and one problem I have with restarting Bioware games is knowing I need to wade through 20 tons of talking to people to get around to actually playing the game. It's cool the first couple of times, but it becomes something of a chore to say walk around and speak to everyone on your ship (if you want to do things right) after every mission, or navigate all the NPCs at the beginning of the first "Knights Of The Old Republic". Even the most awesome dialogue and writing loses something with repetition.
