Bioware and Bethesda both make amazing games. Bioware has several awesome stories under their hat - The KotOR series, Jade Empire, Neverwinter Nights, Baldur's Gate, Mass Effect... Bethesda has the Fallout series and The Elder Scrolls series...
I personally love them both to death, but I'm going to give this one to Bioware. Yes Bethesda's worlds are much more immersive, allowing you to go on random killing sprees just because you want to, but they just don't have the epic storytelling that Bioware does. I'll never forget the first time when Darth Malak revealed to my character his identity as Revan in KotOR, or Kreia's betrayal in KotOR II. Bethesda does have "epic" stories, but I just don't like their storytelling as much as Bioware's.
You may be wondering why there's no "I love them both too much! I can't decide!" option. Indecisiveness is a form of weakness- YOU MUST CHOOSE YOUR FATE.