I own both (my XDs is a Mod.1 with the waffle grip), and love both.
I have XL hands (glove size) - the grips are different, but similar in overall circumference. The XDs grip is longer and skinnier; the Hellcat is more square (wider and shorter).
Fully loaded, the Hellcat is lighter compared to a fully loaded XDs.
I've upgraded both sets of sights - my XDs has Truglo TFX on it, my Hellcat I added a Trijicon rear night sight to go with the factory front night sight. My XDs has a Viridian Reactor R5 instant-on green laser; my Hellcat has an Olight Baldr Mini green laser with tac light.
Shooting-wise, they are VERY similar. So similar, in fact, that in 2019 when the Hellcat was released I didn't buy one, because it was so similar and I couldn't justify the expense, for basically the "same" gun but with more capacity. Then, COVID, riots...2020 in general hit, and when I saw a Hellcat in stock, I took it home. Biggest difference is, I had to change where my strong-hand thumb goes on the Hellcat - I kept hitting the mag release. I changed to putting my support thumb under my strong thumb, and that fixed that. I am likely on the release button on the XDs as well, but it takes a much more deliberate push to release the slide than the Hellcat does. The Hellcat reacts more to hand placement as far as shot placement goes, but that's a function of the shorter (3" versus 3.3") barrel and it is absolutely manageable. Neither is "too" snappy, and with practice you can learn to "time" the rhythm of the gun (fire-slide recoil-reset-fire), and dump a mag accurately on target with ease. I can put a full mag into an 8" plate at 10 yards, no problem. With either gun.
The grip safety is nice, but on a small handgun like these it's pretty hard NOT to hold it the right way so it's more peace of mind than anything. I don't miss it on the Hellcat...but I also appreciate it on the XDs. They're both drop-safe with internal safeties and I carry both IWB in kydex hybrid holsters with good trigger protection.
Teardown and maintenance wise...they may as well be the same gun. Built just about the same; tear down the same; clean the same; reassemble the same.
Less than helpful, I'm sure, but to echo the above statements - shoot both. One will be "right" for you, your hands, and your stance. The other will be close...but one will edge out the other. Personally, I think I prefer the XDs a little bit at the moment, but the capacity and lighter weight (even with the fully loaded 13 rd. mag) sure are nice on the Hellcat. I wear my XDs daily due to the Hellcat starting to pick at my clothing - that grip is AGGRESSIVE - but when I'm wearing non-work clothes...I've got the Hellcat on. If you shoot them well, and are comfortable with them, you can't go wrong with either.