It's like this:
Both good --> UPD is better!
Both bad, top card is worse --> BG is better.
Both bad, 2nd card is worse --> same effect.
Top good, 2nd bad --> same effect.
Top bad, 2nd good --> BG is better.
So UPD is better only when you'll want to keep both cards more often than you'll want to skip the top card. Or if you've got strong top deck control somehow, like with a Druid or something, and you'd like to work from the bottom too.
EDIT: ok, UPD also has a chance to make you get a specific good card after one more +card, and that's difficult to quantify, but less meaningless because it no longer has the "whiff once, whiff forever" design to it.