I have both the S510 (.22) and the HW100 (.177 & .22) so I'll add my 2 cents.
When I first was looking for a PCP I was totally taken by the beauty of the S500 but the lack of a regulator was a concern.
I decided on the HW100 but the .22 was not available. With the lead time being so long so I opted for the .177 with the intent to upgrade to the .22.
Liked the HW100 so much that when the .22 became available I bought one and kept the .177. I've been very happy with them.
Fast forward 10 years and I finally got around to getting the S510 now that they come with a factory installed regulator. The S510 is a gorgeous airgun, well made and a pleasure to shoot.
That being said, I feel that the HW100 is a better airgun. Accuracy, quality, and trigger are pretty much equivalent between the two. IMHO, The Weihrauch magazine system is far better. The AA magazine seems to be an add-on the the S510, it's flimsy/delicate and a nuisance to load.
Ergonomics is a personal thing but I find the balance and feel of the HW100 to be better for me. The walnut stock fits me well (I'm average size - 5' 10", 180 pounds) where the weird cheek piece on the Air Arms walnut stock wouldn't work no matter what I tried. I ended up having to buy an adjustable laminated stock to be able to shoot the S510.
Don't think you would be disappointed with either PCP, they are both fine airguns. I'd strongly recommend, if at all possible, that you handle/try any airgun/scope combination before purchasing.
Just a note, the HW100 and S510 are of similar design vintage, with the S510 being based on older (12-15 year old?) technology. A lot has happened with airgun design in the past few years, you might want to look at some current models before making a decision. Daystate, FX, Feinwerkbau, and Rapid Air Works make some great airguns and there are many others that I'm not familiar with.
Specifically, the FWB field target rifles are outstanding (and expensive).
Good luck with your purchase!
Cheers!