It depends
Like anything else, it depends.
For manufacturer, you usually get what you pay for. The Avery guns that I have used are very nice.
As to 2X vs 3X I think that also depends.
I built a slow build RV6 with a borrowed 3X rivet gun and it worked just fine. But, for 99% of the riveting it can get a little heavy and unweildy, at least for me. If you are building a slow build, there are some larger rivets that a 3X will do better. For a QB, I don't think there are any rivets that require a 3X gun.
I got a 2X for the QB RV8 and it worked great for all rivets. I just put a small regulator the base of the gun and made marks at two air pressure settings, one for #4 rivets and one for #3 rivets. The 2X gun is smaller and lighter, which is great for me since I do a lot of riveting by myself.
If a nearby builder has a 3X gun you can borrow, you might consider getting the 2X and trading back and forth as needed, or vice versa.
I would recommend trying them out. Seek out some local builders and I bet they'll let you try out what ever they have so you can get a better idea what you are most comfortable with.