In fact, there are two questions: AV versus first past the post, and then AV versus PR. The question of whether it is proportional.... It's not a proportional system, so I'll just address the first question, which is AV versus first past the post.
It's a single-member district, so it's very similar. The differences are not big. The only difference is, of course, that you rank-order your preferences, and a candidate has to receive a majority of support in their first vote or second vote or third vote. The major difference is that a party or candidate that is the second choice of many people gets more support and is more likely to win.
There have been some simulations. Basically, the system is not too different from first past the post, but a party that is the second choice of many would get more seats. That would be the biggest difference. It's up to you to decide which is the party that is the second choice in a given context, and then you'll see which party is most likely to be favoured at a given point in time.
That's the main difference. It's more legitimate, in the sense that every candidate who is elected gets at least 50% of the vote. In my view, that's more legitimate. It is still not proportional and so on in many different aspects, but it is, in my view, more acceptable.