You're not likely to find many that have tried both and these sorts of "I have and like X" replies are what you're probably going to get. We have Zunsports but have never tried the Rennlines. The latter sound like they would be more secure but we've never had our Zunsports fall out except with a collision and even then we only had one of the 3 pop out on the driver's side (Boxster).
I have the Zunsports on my 981C-Base w/ parking sensors. The middle vane does not fit well, at all. I *think* this may be specific to the Base w/ sensors. In fact, I had to make some serious cutting down to get it in reasonably well, which doesn't look great but will save the radiators on an upcoming roadtrip of a few thousand miles. (I know some may say I fitted it wrong, but I didn't. Honestly!)
I contacted Zunsport and they were very good, to be fair. But I think I need to go there to get the issue resolved. I am in the same country, but about 4hrs away, so it remains unresolved.
There is also some strain in the corners of the plastic Porsche grille/opening, evident from the fitment of the bottom vane. This does seem to fit "well", but the tightness of the fit, necessary to hold it in place, is the cause.
With a properly fitting middle piece, I'd give Zunsport 8/10. As it is, 4/10. When I get the time, I think removing the bumper, cutting some Aluminium grille and fitting from behind is going to be the best option.