Well, it's kind of apples and oranges, because P3D has essentially been in development since 1982 (given that it's built on top of the FSX code, which was built off the previous incarnations of MSFS), while this is a new title which hasn't even been released, and is building off a sim that's only a couple of years old. So P3D absolutely blows it away when it comes to feature set, available expansions, and so on.
But in the areas where they do match, AeroflyFS 2 has dramatically faster loading, supports VR without an add-on, has much sharper graphics in VR mode (I can read the instruments without leaning in, like I have to do in P3D/FlyInside and DCS) and at least on my system it has much smoother frame rates despite the extra graphic detail.
I think any serious sim fan would want both. FSX or P3D when you want the whole world, sophisticated add-ons for weather etc, thousands of add-on planes and scenery, etc. AeroflyFS2 for beautiful scenic flying in the supported areas, amazing VR support, aerobatic flying (the Pitts and Extra are crazy fun to fly, particularly with the sense of speed you get), and the other stuff that's there now.
And buying it now (1) helps support the company's efforts to build up the sim's capabilities, and (2) gives you a chance to try it early and help influence its development to hopefully include the stuff you'd like to see down the road.
Back in the 1990s I was the flight sim columnist for Computer Gaming World magazine. And there was enough new stuff that I could write about new sims
every month! This year is the first in forever that I've actually seen more than one new sim released (this and the
Combat Air Patrol 2 Harrier sim, also in early access, and which just released its first VR test version today).
Rather than insisting on comparing new sims or flexing our rivet counter muscles, we should be overjoyed to see some activity in the market, and doing our best to help make these cool early access sims successful so they can get even better!