I've recently bought both games, but haven't played more than 8 minutes of horizon due to it locking up early in the game. Today was the first time I tried playing it. The opponents seemed to rubberband during the one race that I played. I bought this hoping there is a drifting aspect to it. I haven't seen a drifting event in 7 yet. It's not to say it doesn't exist, but I'm pretty sure there isn't one in the game at this point.
Forza 7 runs very well on my system at 1080p, I don't have a 4k monitor but the game seemed to indicate my video card was enough to run it at that resolution.
My impression of the games is forza 7 is more like a simulation/arcade and horizon 3 leans more towards a burnout type game(arcady). In forza 7 I haven't seen any opponents make a crazy comeback during the races. One thing about the game I don't care for is the race modification cards, and the loot boxes that are purchased with race winnings. Early in the game, I spent 300k on a lucky car crate and won a 60k car in it. This really made me mad so I went offline, opened the fearless cheat engine table and modified the script to give me 120 million in the game. Then bought a few hundred of various crates and then used a macro to open them all. I don't care for the online play and am willing to take the risk if I get banned, so far I haven't been. I haven't sold anything in the online auction to draw attention to myself however and ruin the online economy if they even care about it at this point.
I will probably try 3 again later this weekend. I will update this once I play more than a couple minutes.
EDit:
I was able to get horizon to play after trying another time. I played a few races. Graphics wise there is a difference between the two. Horizon is very similar to the Burnout Series of games, where you challenge people and gain fans. I do think it would be a more fun casual game though. For 30 dollars or whatever it is on sale for, I'd say go for it. I'm not sure which I would choose if they were the same price though.