When she said "we", I took that to me "we" (she and John). I think what it ended up meaning is "we can't both go or who will stop Skynet?" What that boils down to for me is that there's work to do and skipping ahead to find Cameron isn't part of that. But he went and she didn't try to stop him and it bothers me a lot.

John's a determined kid ("he's a strong boy with strong opinions" - "don't know where he gets that from") and I have no doubt he was going whether she did or not. But it was the worst possible moment for Sarah to let him go without a fight. I know the whole thing played out in like 20 seconds, but there was time enough to say more before giving up.