I've got a pickup and I personally think that it gets slightly worse gas mileage than a comparable rabbit. I'm getting about 37-39 with a 1.6TD. I've got a few mods (bigger downpipe, GTD injectors, LDA adjustments, more boost) and I drive with somewhat of a lead foot, but I haven't noticed a real drop in mileage as a result.
Regardless, I've heard stories of people getting in the high 40's for gas mileage in their rabbits... I've never seen it on mine, even when it had a 1.5 in it.
I would think that a rabbit truck would do better with a cap on it, but I can't say for sure. My truck doesn't have the front air dam either. I'm wondering if that might make a minor difference.
Anyway, if I were you, I'd make the call based on what you "need". I love my truck to death. It hauls a ton of stuff, gets good mileage, rides pretty well and turns heads. For me the utility of having the truck outweighs the few mpg I'm not getting...(plus, 38 mpg ain't that shabby to begin with).
The downside to the truck is that it has the SMALLEST interior of any VW I've owned...even when compared to my Ghia coupe I had. If you've got passengers, count on putting everything in the bed...very little room behind the seats. I'm 5'11 and I fit fine in it, but others have complained that the trucks are too confining for them.
The other unfortunate part is that rabbit trucks (especially the diesels) seem to bring a TON of cash these days. I've seen some 1.6NA trucks go for $5k and UP on ebay. Whoa. 4-dr rabbits might be cheaper to find.