While there are nuances and skills to oval racing and bangin' fenders that the NASCAR guys probably won't get credited for on this forum, I think the F1 driver is better suited to transitioning to NASCAR than vice versa. Not that either transition would immediately result in a championship season.
The physical demands on modern F1 drivers are phenomenal. The G loads the cars can generate are bordering on ground-hugging fighter jets. I think the NASCAR guy would have a lot of work to do just to get up to speed physically. In his F1 "test," Gordon was astounded by the performance (monstrous deceleration) provided by F1 carbon-carbon brakes. Also the experienced road-racer in a road-racing world should have a bigger advantage in technical skills than the oval racer in an oval world.
Another thing about that Gordon/Montoya car swap in 2003 is that while Gordon was within 1.3 seconds of Montoya's pace in the F1 car, Montoya was driving like a grandma that day, ~6 seconds off the previous year's pole position time. Unless they had changed the track configuration for that exhibition, apparently Montoya was driving at 7/10ths or something. Granted, Gordon had only a handful of laps in the car, and was petrified that he was going to destroy it and was tiptoeing around the track in certain spots, but I don't think you can assume that Gordon would just leap into an F1 car and be competitive without *a lot* of work.
Above all else, in practical terms, you're more likely to look mediocre to downright horrible in F1, because there are, at best, 2-3 teams that are realistic championship contenders in any given year in F1. So you better at least be highly competitive with your teammate from the beginning of the year, otherwise the rumors are going to start, and you're going to get kicked out midseason if it gets bad enough, e.g. Michael Andretti, Scott Speed, Sebastian Bourdais, etc. The learning curve is steep and the tolerance for failure is very small.
Nascar seems to be more willing to stick with a guy for a while, have less of a hair trigger when it comes to kicking drivers to the curb, sponsors and brands are committed to and have strong multi-year relationships with drivers, etc. You seem to have more of a fighting chance to get experience and prove your mettle before they guillotine you in Nascar.