Its really not fair to compare the 2 as they make different types of games and each dev does an excelent job in their respective bailiwicks.It all comes down to what type of game you prefer to play. Personally I prefer Bioware over Blizzard as their games focus more on story and character development than Bilzzard's do.
Black Isle/Interplay and Bioware all kinda run together in the Bauldars Gate era as they collaberated a lot on each others projects. Biowares Infinity engine almost made them bigger as a engine provider than a developer at the time. Biowares games always seemed better to me when Black Isle had some input on them.
Peronally Id like to see Bioware absorb some of the remanants of Black Isle from Obsidian and Trokia to expand their studio.
