This is all my opinion, for what it is worth - and little background to justify my opinion:
I have been on the inside of AA S400 (my own) and the HW100 and a DS huntsman. Grew up in a machine shop, and do airgun repair.
- Engineering: 1 HW, 2 DS, 3AA. Particularly impressed with the engineering and thought that went into the HW mechanisms. (There is a lot to say here!) I found the AA lacking. Couple of issues I had to fix. Burr on the transfer port, and the main valve concentricity was out, causing it to have a small leak at lower pressures. (This was 10 years ago - ymmv)
- Fit and Finish: 1AA, 2 DS, 3HW. The walnut on my AA, with Rosewood end caps, and deep blueing, I think this may tie with a Daystate. I find the HW to be a bit heavy, and clunky in comparison.
- Accuracy - I'm pretty sure all 3 can be tuned to Sub MOA @ 50
- Reliability - I can only mention the types of issues I have seen, I could be the only one to ever have a main valve break. Don't know. - HW seen mostly leaks, and a broken main valve. DS mostly leaks, and a side lever action breakdown.
Personally I'm also torn between the DS and HW.
I'd probably choose the HW, as it has been around a while, and has quite an established following, to the extent that there are quite a few after market options available. Titanium reservoirs to make it lighter, and it comes in a few 'flavors', Carbine, Thumb hole. The mag also does not stick up above the breech, and has 14 rounds. Keeps the scoping area 'clean'. I would expect the longer barrel (430mm vs 600mm)
may be more efficient, Or if hunting - less maneuverable, then get the Carbine. The insides are built like a tank, and I like to tinker.
If I was looking for a light beauty run'n grab'n hunt - I'd go with the DS.
As you said, all great guns - have fun with the one you choose
ET