It could always turn out that one of the objections is instantly recognized by the entire group as a fatal flaw in "foo" and the group will then turn to a discussion of the merits (and demerits) of "bar" instead.
Getting everyone to agree about what are the negative properties of each proposal is key. Without judgement. The engineering is then about figuring out if the negatives of proposal bar are worse than the negatives of foo. Often it's not the better proposal that wins, but the proposal that has fewest (or no) showstopping negatives..