There is no comparison. Bioware by a mile.
They single-handedly revived the (western) RPG genre with Baldur's Gate and have continued to follow up with titles such as NWN and KOTOR.
I am also looking forward to Dragon Age.

They do have some blots on their copybook, however (I'm looking at you Icewind Dale).

On the Bethesda side, Elder Scrolls was OK, but nowhere near as immersive (I own and have finished the entire series).

Bioware is the best. Mass Effect, Jade Empire and Never Winter Nights series FTW.

I hated Fallout series and Elder Scrolls series was terrible.

Black Isle!

Bioware by far. Bethesda makes games that just seem RPGs and the storylines are always boring.

I like Bestheda and enjoyed Oblivion but I must say Bioware is better. You can not compare Baldur's Gate to Oblivion, Baldur's will always win. This may be a bit of bias though because I used to play a fair share of DnD.

I don't even see how this can be a question. Bethesda has never made a good game IMO. They cut every corner they can, and end up with a massive amount of boring nothingness. It has been one of the biggest questions on my mind for years, as to why people care so much about this company.

Bioware on the other hand is freaking briliant. They are stil corner cutters, but to a much smaller extent. A lot of people claim gameplay is the most important aspect of gaming, and when it comes to WRPG's these guys are the king in that catagory. From a technical stanpoint Bioware also sales way above. I love the voice acting in their games, Bethesda games always have monotone VO's, while Bioware does a great job with VO's.

I know I may sound harsh, but it is like asking who makes better JRPG's Square-Enix or Koei?

Black Isle.  Then Bioware.

Holy shit, I hadn't read any other posts here when I posted for Black Isle.  Too late I was.

I fellow compatriot! Long live Planescape!!

Bethesda for me, Mass Effect was way to short for me only a good 30 hours and I already finished it.