Excellent question Nate!! I bought a Makita 4340 D-handle two years ago, and went through the same dilemma. I ended up buying the d-handle over the barrel grip because I was informed that the barrel grip was more for industrial use (was not explained why though), and because I read that the d-handle was supposed to be more comfortable (which seems to clash with the explanation that the barrel grip was for industrial applications where you would use the tool all day). I've noticed very few manufacturers produce the barrel-grip style, but the ones that are produced tend to be the top-of-the-line model, and those are in addition to an otherwise identical top-handle version. I'm aware that Makita, Bosch, Metabo, and Festool make barrel grips. Someone also said something about barrel grips being easier to maneuver....
What kind of jigsaw are you using right now? What have your previous jigsaw experiences been?
By the way, I'm VERY pleased with the Makita 4340 design, and if Festool was NAINA, I'd recommend it in a heartbeat over the Bosch 1590. I have used both pretty extensively, and the Makita seems smaller and more nimble, less cumbersome, has the VERY handy LED light that illuminates the cut line, and has exceptionally low vibration. I would be comfortable using it all day if I had to. That's just my opinion though, everybody is going to interact with their tools slightly differently..... My only complaint would be lack of provision for dust extraction, and Plug-It cord, which could be said about most non-Festool saws....