We've stayed in Paris in August with and without AC. If it's hot, you probably won't like it, but an inexpensive circulating fan does wonders. You can either gift it to the apartment where you're staying when you leave or trash it. I guess I'd ask in advance if the owner would welcome it or see it as left-behind trash.
We've also been in Paris when the weather in August was mild enough that no fan was needed. My wife begins finding the heat intolerable before I do, so obviously it's a personal judgement call. Where you're from might enter into the equation but I'm unsure about that. We're from Florida but as soon as it gets hot, we turn on the AC so we're not accustomed to sitting around or trying to sleep when it's hot inside.
I'd vote for the place with the solid reviews (and a circulating fan). My wife would probably vote for the other one (depending on what those mixed reviews say).