The other day I was asked if a letter should read: “Staff members at the Local Planning Council, with whom we’ve worked for over ten years” or “Staff members at the Local Planning Council, which we have worked with for over ten years.” My response was something people don’t want to hear. I said that there’s no clear answer. Both are correct.
“But which is more correct?” I was asked. In this case, it was a high-stakes bet between two office-mates, neither of whom cared to back down. Again, I said, both are fine.