Better in terms of... what exactly?
If it's speed, technology, strategy, driver skill or any number of other metrics then it's pretty clearly F1. I'm not saying NASCAR drivers are unskilled or anything, quite the contrary. I just don't think they, or their cars, are on anything like the level you see in F1.
I strongly suspect (unless you've got the money to watch from a yacht in Monaco or something) that NASCAR is probably the better live sporting experience though. I've never actually been to a NASCAR race, but I've been to the track for the Australian F1 GP a few times and it's pretty rubbish to watch live. You can invariably only see one small section of the track, so chances are if something cool happens it won't be where you can see it. It's a sport where splits and strategy and having the full picture matter a lot to your enjoyment of the race, and that's tough to keep up with live too. And in recent years the cars don't even sound as cool any more.