For the benefit of future readers, these choices are not mutually exclusive. You can use the TP-Link service to control your HS100 via Google Assistant and also have it as an openHAB item (that doesn’t need to be visible to Google Assistant). I do exactly this, because I already had my TP-Link switches in GA before I got into openHAB and I didn’t see a good reason to change everything over. If I had been in openHAB first, I might have just relied on the OH service.
I believe the main benefit of using the OH service would be that you can deny your HS100 access to the Internet (since OH communicates directly with them). But if you do that, I don’t think you’ll be able to get firmware updates.
For me personally, its more about avoiding to many accounts, one for xiaomi, one for tplink, one for yeelight, one for darksky, and the list can keep going for a while … thats for me the only reason.