Firstly, you just can't reliably compare eras. Full-stop.
Secondly, I'm assuming you're just talking about batsmen - otherwise arguably Garry Sobers was a better cricketer than either Bradman or Smith. Even with the bat he was pretty awesome (my uncle still talks about his double century in Melbourne, which apparently Bradman himself called possibly the finest innings on Australian soil). Similarly, Kallis would have a fair claim to cricketing greatness.
With those caveats aside, I'm sure I've mentioned a few times before that Smith made 1,000 Test runs in a calendar year at a 70+ average four years in a row, which is extremely impressive. He's made runs on a variety of different pitches in countries around the world, and has saved Australia more times than they deserve. And even with a shortened year this year on current form he's more likely to make 1,000 runs than anyone else in the team. I think it's reasonable to compare him with Bradman.
EDIT: Trivia - when I was on a tour of Lord's Cricket Ground a few years ago, they hurried us through the Long Room because it was busy being set up for Sobers' 80th birthday party...