Indycar is big improvement:
-Better feedback through FFB
-Corner entry takes more practice as you can't just stamp on brakes and turn it in because it will lockup front tires
-Brakes also feel much more sensitive to unloaded tires and I had plenty of single-tire lockups in Mid-Ohio. Probably even managed to lock every tire on a single lap without spinning/crashing out.
-There's bit more progressive feel to tire slip and it's not instant spin everytime you hit throttle too hard. Same applies to braking, feels like there's more than just two states of tire rotation; spinning and stopped. That is pretty easy to notice when you get rear-lockup under braking and if it's roughly on the threshold and just one tire, car will just nudge a little sideways and settle back in as you ease off from brakes.
-Old setups are junk, too understeery and you have to run much less downforce because the drag has been increased by a huge amount.
Cadillac (oh boy....)
-It's essentially soft-Corvette (Cadillac suspension, Corvette engine that feels weak)
-Default setup is absolutely horrible and while you can make significant improvement by messing around with the differential, you better get used to dealing with massive lift-off oversteer and sudden spins at low speeds.
-Tires feel little weird, seem to have very high optimal slip-angle and "sliding" through corners is significantly faster than keeping it clean. The angle is significant enough that your brain keeps screaming "Counter-steer! Counter-steer!", making it pretty hard to do consistently with limited practice.
-Steering also feels little bit dead on the center, but you'll get used to it.