rnk It really depends on what you're after. Many of the differences are layout or more so conception than being 'better/worse'
There is a lot of options for extensions and such, although there are some common things that arent available/easy on flarum yet.
A common request by my users has been chat functionality, there is Neon Chat. although it doesnt quite match what me or my users are quite after. The standard forum experience is quite good in my experience.
Another thing that you might find quite different is the way tags are dealt with, I believe most xenforo sites and other forums focus on Categories, which cant quite be emulated via flarum in exactly the same way imo, even if you use the categories extension. Although a better admin/dev might be able to make their site feel closer to what I think of.
In my experience flarum has been very fast, although when I first set up my site it had huge slowdown and loading issues. It has improved with updates.
This discussion site has quite a few extensions on. My site does as well, I thought I could attach an image but I cant directly so the link will do: SheepishPatio
Maybe have a tab open with a couple XenForo sites you like and a couple flarum ones, think of mine as what someone who tweaks things a little can do and this site as one with quite a few extensions. If anyone else has a site to add for comparison feel free to reply i'm sure it'll be handy 🙂