I believe the S500XS is regulated. I have a Daystate Revere in .22, and previously owned an AA S510XS. They are both excellent rIfles, and the more accurate would be luck of the draw, the better barrel. Neither are especially owner friendly with respect to tuning, but if it delivers what you want, you should be very satisfied with either one.
The power adjustment on the AA is a bit too gimmicky for my liking. It will adjust power, but only by restricting the TP. If you want a well balanced tune at a given power level, then the reg set point and hammer tension should be adjusted to the desired tune. Just my opinion. They are both great, classic sporter style rifles, of which we have too few today. Seems the market wants high power, tactical styling, and heavy slugs. IMO, these are not the qualities that have built the loyal following for air rifles of the type these two so well represent.