Bridge calls for two very specific resources, while grocer lets you use any resources. Usually you'll have a couple extra goods lying around from earlier in the game. I've seen people fawning over bridge...but I don't see it. Convince me otherwise!
I feel like, in those two short lines, you've kind of exhausted the topic.
The Bridge has a better VP return at the cost of specific inputs. The Grocer has a reduced return counterbalanced by open input. They are both powerful buildings. They are both round-3 buildings, which means they will be used only once by their owners in most games, and on rare occasion, twice. Players will rarely be choosing between them, and if they were, it would be based, as you say, on their own goods as well as on the usual turn-order implications.
I frankly don't see how one is, in some non-situational, general way, more powerful than the other. They are both strong endgame tiles.