i would say that in the latests builds of Vray there's not so much rendertime difference between 2d and 3d disp. i've read somewhere that 3d is the way to go as it have been improved a lot in latest builds.
note that the Vraydispmodifier, in 3d mode, have pretty the same settings than the global displace settings in vray (via map).
if you've several materials on your object (multi or fusion) and want the displace only on one of them, then you can't use the modifier, as it impacts the whole object.
If you use the map, you'll be able to apply displace on only one material of the object.
The modifier allows you, too, to have different displace settings in your scene. If Using maps, they will all use then same global displace settings. Hope it helps !