I have both non-OSP XD-s (Mod.1 waffle grip), and a non-OSP Hellcat. Yes, the grips are quite different - fractions of an inch make a huge difference in feel. The XD-s is notably longer (front-to-back) in the grip compared to the Hellcat, which feels more 'square'. They shoot (for me, anyway) nearly identically, which actually delayed my Hellcat purchase quite a while because I couldn't rationalize buying such a "similar" gun strictly for capacity. Then, 2020...and....well....
The choice between the two for anyone, will literally be 'how does it feel in my hand'. If you shoot one well, chances are you'll shoot the other one just the same. The only adjustment I have to make when I switch, is making sure I don't tickle the mag release on the Hellcat or it'll drop the mag. The XD-s requires a concentrated effort to get it to drop. But that's about the only difference for me.