Does the pixel count for cars differ between both versions?
What? Do you mean the polygon count? I'm pretty sure that they won't--higher poly counts aren't really going to raise graphical fidelity for cars much anymore. I'm sure that they'll be slightly higher than GT Sport (mainly on small things like badges/tires/other small details/etc.). The effort is mainly going to be going into lightning systems, textures, and environments.
And draw distance plus LOD pop in. It's still there in GT Sport if you look for it. Signs popping in, even buildings in the far distance. There is also a visible LOD change in car detail when they get closer but harder to spot.
The main effort is going to be in reflections, shadows and particle effects. And those are all part of the lighting system as well. Plus more 'interactivity' with track objects, better physics in collisions with track objects. (Pretty silly when a little cone launches a car in the air!)
Plus more effort in online physics is needed. GT Sport often has weird looking collisions due to lag. You don't actually collide with other cars on your system, you see the result of the collision of your car with your opponent as it happened on their console. GT Sport uses forward prediction, the more lag between you and an opponent, the more 'displaced' your cars are on each others console.
So even if your opponent behind you brakes on time, that event might reach you too late and your opponents car on your console runs into you from behind, you get bumped forward (likely now not making the corner anymore) On their screen they never touched you and see your car suddenly jump forward.
It's still the better option than doing the entire race on the server which means everyone has input lag. They it is set up in GT Sport is that you have real time control of your car without any fluctuations, yet that means all other cars have to be forward predicted to stay in sync with you. This can easily be several car lengths when playing against someone from thousands of miles away. More fine tuning is needed in how collisions are handled to avoid the exaggerated 'hammer' effect with lag. Playing against South America from Canada can easily mean 200ms of round trip data difference or more.
In single player you of course don't have to deal with that. Yet there PD made AI cars very 'heavy'. You can't bash them around as much as online opponents, and are too easy to use as 'brakes' and corner 'aids'. Which is probably why people are so terrible at racing when they get online. The single player allows far too much bad driving. Even encourages it as 'diving' into the corner is nearly always rewarded in single player. The AI simply gets out of your way.
Which is another area PD really needs to work on. The penalty system online, implementing actual racing rules. Atm it's far too simple. If contact -> look who came out worse -> penalize other car. Since 'who came out worse' is often 'calculated' as who loses speed, the perp usually gets away with ramming you.
There are still a lot of improvements possible!