I respect Bruce for what he does -- he's a hustler and has created a brand that resonates with many fans. He brings eyes to the sport and is generally well respected.
With that said, Michael Buffer by a landslide. Bruce Buffer's voice makes my ears bleed (respectfully). Hell, even when I used to listen to his weekly show on the Sherdog Radio Network where he'd humble-brag about fighting Frank Trigg and other shit I was not a fan. My old coach used to fight in the UFC, and he told me that when he spoke to Buffer, he more or less came across like a dude that was a fighter -- very douchey, but at the same time, I kind of admire how seriously he takes the job. Joe Martinez is the best voice in combat sports in my opinion. I'd prefer if the money spent on Buffer instead went to the fighters.
With that said, I'm no dummy -- Buffer probably does more good for the UFC than bad. His success may be gravy-training off of his brother, but I say don't hate the player, hate the game. Lastly, I should add... I may be alone on this even among Bruce's fans... but I fucking love Bruce Buffer's suits. They are very fucking fun in my opinion and unique. I'll take his suits over a black tux any day, though I acknoledge that perhaps I'm out of my mind with this haha
TLDR: Michael Buffer >>>> Bruce Buffer