I have run both and the 32/36 delievers good power and good milage. The 38 can deliver very good power (especially when coupled with a good ignition and headers) but is near impossible IMO to get good milage out of. But on another note my roommate's 38 gets about 16+ MPG on his 4.2 CJ7 (but he has a 5th gear and a lot less weight) while I'm struggling to get 15 MPG. Good luck!
According to some of the Toy shops the 32/36 is not big enough for the 2F and you won't see a Webers full potential unless you use the 38/38. That's what I found out prior to running mine. And yes it made a huge difference when combined with a header. But now I have switched to a V8.
I have my 38/38 (complete) for sale if your interested.
I'm running a 32DIR Weber on my F. It is on the small side I guess, but I'm very happy with it, and it has more airflow than the single barrel it used to have. The engine is otherwise stock and I rarely find I need to open the secondary barrel. Personally I'd not buy a 38 synchronous openning carb if fuel economy matters to you. Stick with the 32/36 progressive carb, set the primary barrel slightly weak, and the secondary richer for when you want WOT. On the other hand, if you want more power and drive such that you use it, your economy will suffer either way. By the way, I average 15mpg now, but when I was setting the carb up it bettered 17mpg on the hwy. Only problem was it was so lean it didn't have enough power to spin the tires on gravel!!! :-[