AV, or IRV (Instant Runoff Voting), is arguably the worst form of preferential voting in existence. Take this example of IRV totals for four candidates ordered by preference:
30% a; b; c; d
35% c; b; d; a
20% d; b; a; c
15% b; d; c; a
Given the voting breakdown, most people would agree that candidate b should win, because no candidate got a majority of first preference votes and b got a sweeping majority of second preference votes. However, under IRV, b is eliminated immediately, because b received the fewest first preference votes.
tr;dr: IRV is terrible, and has a history of being adopted in states and cities in the US only to be abolished not long after, because of the bad electoral outcomes it produces. If you're going to pick a preferential voting system, it ought to be one that at the very least preserves the Condorcet winner; IRV is not that system.