I want to be fair to Harem Collector. No Moshing's team is more gameplay focused then us. I'd even say that Harem Collector has more in-deph and developed mechanics then almost any other Hentai RPG Maker game you can find. We double down on the story telling depth in a way they don't really focus on. I think you'll find that the approach is meaningfully different. That said, the similarities are pretty much abound between the two games. They are both about a guy who develops a harem of slaves in a fantasy setting, after all.
I played Harem Collector until August.
It basically has a lot more quests than Harem, slightly better combat system, better music IMO
Dialogue wise Harem collector is more funnier, amusing, cheerful, while Harem RPG is more straight to the point. If you want to play something passionate and serious, Harem RPG is more to your liking, while Harem Collector is more about having fun, etc.
Harem RPG is more well written though IMO, and has a better story.
Character development wise they are about even. Harem RPG has really upped it's game in instilling a personality toward most of the known girls. Both games have a lot of worlds to explore, albeit Harem Collector might have more.
IIRC Harem collector will have about 152 women to enslave, so I don't think Harem RPG is going to go near that amount to enslave.
Sex scenes, Harem RPG has the edge, as I'm not a fan of the 2d style of Harem collector, and the 3d style of it basically feels like a hologram with a dildo fucking the girls.
Objectively Harem collector has some few attributes ahead of Harem RPG, while the latter has a few attributes ahead of Harem RPG.
If you have a lot of time on your hands and want to play a light style comedy game with RPG fighting elements, Harem collector is the game you should play. Unfortunately I just couldn't get into it after playing it for several days.