Top speed: Indy car by a lot. Both cars run at Indianapolis, and the indy cars qualify at 224 mph. Nascar at 179 mph. Indy cars are lighter and have about the same horsepower.
harder to drive: Sort of like asking who are better athletes - sprinters or LD runners. Very different styles. Nascar cars are 3x the weight and have primitive technology. Indy cars are like a scalpel, nascar like an axe. Hard to drive 3000 lb Nascar racecar at 200 mph with terrible aerodynamics - the car is like a high powered brick. it slides all over the place. But you are only turning left.
Indy car is like a racing spike - light, fast, more aerodynamic. Much more precise, but you have to brake, accelerate, turn both ways, etc.
nascar has much more money in it. Indy Car has almost folded a few times.
nascar is more popular everywhere. Nascar fans used to be heavily focused in the South, but that has moderated a bit. Lots of stereotypes of drunken redneck nascar fans. Some of that is true, but the fanbase is widening.
Indy car doesn't really have a wide fanbase. Open wheel is just not a big deal here - sort of like cross country. Nascar is probably more blue collar.