If you can save 500 then buy S510 and don't worry about it. They are as mentioned very similar in performance. Build quality is about the same too. Daystate mechanical platform is not magic also.
Air Arms will be also a more consistent rifle to shoot, shot to shot. As the hammer is ridding on a guide rather then like on a Daystate inside the lower receiver tube. So in that sense S510 is a more well developed gun. But Regal has a better profile wood stock, and is lighter, but more louder actually without ldc. Air Arms vents its shroud from the receiver. Adjustable power on S510, which is a transfer port choke is not a big deal, best to shoot at high power all the time. Many folks think it is not necessary, as a sales point sure sounds good though.
Accuracy wise about the same, both use Walther premium barrels. So guns will be as accurate as you are. If you like a Daystate, like it seems you do buy it. Or buy 2 guns and just keep one that you like. All this is subjective opinion btw but hammer/valve design wise S510 has a slight advantage.
I have owned both btw, an S510 and a Huntsman, and I loved them both, still don't know why I have sold them. If I had the money to buy them again, I would buy both again
p.s. If you can get a good deal on a Wolverine, that would be a better step up from both of these.
