Barrel grip is handy for working from under neath/the back. Also your closer to your piece your cutting so you feel like you have more control!
Down side to barrel grip is either ON or OFF and it can be a little dangerous if you hit something it bounces you about you struggle to find the off switch and by the time you have while its bouncing around your work piece is damaged.
This leads me to say that the advantage to the D handle is it has both functions either Permanent ON or you just pulling the trigger to hold it on. I dont know about the carvex but most Jigsaw the triggers are pressure sensitive so the more you pull the trigger the faster it goes giving you more control.
I personally like the barrel grip as it feels nicer. Also I think the carvex D handle version looks and feels very cheap especially the square like trigger doesnt seem to be in the right place. I think festool made the Carvex barrel grip and then went OH DAM IT we forgot to make a D-handle version quick just bang a curved handle with a trigger and that will do.
JMB