Jacques Villeneuve has already settled that.
F1 lighter and quicker.
Indy bigger and faster.
He prefers F1.
Except it's been shown conclusively that indy cars aren't as fast. There are several tracks that have had races in both codes. F1 has been 6 seconds faster on such tracks. Indy cars have a higher output but weigh 27% more and handle like pigs compared to F1 cars.
I'd be interested in Nigel Mansell or Juan Pablo Montoya or Alex Zenardis views, or indeed Seb Bourdais who, despite having been an also ran in F1 seems to have done rather well in the US.