I'll give a brief take. I've played GT3, 4, 5, and 6. Forza 4, Horizon, and recently 6. Forza 6 is fairly new to me, as I bought it was an XBox One S bundle; I don't have a PS4 yet and may get one depending on how GT Sport looks.
First of all the AI. It's always been a weak point in GT, and I think Forza 4 was better at it. I'm mixed on Forza 6 and the Drivatar system. The technology is impressive--as is the number of cars on track--but thus far my F6 races have consisting of stages: 1) Try to get through the pack of idiot AI smashing into each other (reminding me of some bad GT Online experience). 2) Get clear and catch up to the lead three of four. 3) Have a reasonably decent race for the last lap or two. I do like the ability to adjust the AI's competitive level, though.
The sound. Forza's cars sound fantastic. GT still really needs to improve in that area.
The environment. Each have some great and a few not so great courses, but both excel at creating impressive environments. Personally, I like very long, complex tracks as well as night races and rain. Forza 4 didn't have weather nor night, so I kept returning to GT6. Forza 6 has a few tracks available at night or in rain, but no rainy nights. That said, Forza 6's puddles of water which can cause hydroplaning is impressive. But there's nothing quite like the feel of seeing the sky lighten and dawn approach during a long race, which GT6 has.
The feel and control. Again, both have their quirks and strengths. I use controllers rather than a wheel and find the XBox's triggers for acceleration and braking much easier for fine control and doing things like left foot braking and rotating the car with pedals. Both series have some not-too-realistic elements, but both also are good at making players learn that driving a car quickly around the track is much more complex than flooring an accelerator.
Microtransactions. I hope GT doesn't delve into this, at least more than it already has, but I find it a major irritation with Forza. When you look through the car lists you'll see a lot are marked "DLC" as they're part of add on packs, as is a track in F6. And community/online has a special section for those who bought the "VIP" package. F6's layout also has two tabs committed to the Porsche and Nascar pack add ons. With all the Forzas I've played there's been this continual sense of being "nickel and dimed."
Overall gameplay. This is tough, as there are things I like and dislike about each. I will say that if I hadn't been fairly disappointed by GT5 before the patches I doubt I'd have played Forza. While GT6 is a definite improvement and GT Sport has some things that look interesting, I don't have a lot of confidence that Polyphony realizes part of GT3 and 4's greatness was offering players a tremendous amount of things to do, having a lot of replay value, and a sense of progression. In general I sometimes feel that Polyphony has forgotten GT is a game, while Forza has not. That said, when GT gets something right it's often unmatched, and I think playing GT games improved my understanding of how a car behaves.
So this is turning out to be much more than I planned on typing. To sum up I'll say don't "jump ship" and wait to see what GT Sport turns out to be. But if you have an XBox One or plan on getting one, Forza is certainly worth playing and has a lot of offer.
That's my plan, anyway. For now I'll play Forza 6 along with Dirt Rally and possibly fire up GT6 once in a while to see what's new. I'll consider getting Forza 7 and GT Sport when they come out, but I'll likely hold off on both for a bit to see if I like what they offer. Between the two I'm most hopeful for GT Sport, as I'd really like to see Polyphony make a return to greatness.
.
Are physics and handling simulator like.
