I wasn't paying attention to how long ago this topic appeared when I wrote this. I am sure this is too late but will leave it for others that may revisit this topic down the road.

These games are complementary in a collection.
One is old school, kick ass, fun shooting, great flowing pinball. The other is state of the art, ground breaking, mind bending, exhilarating pinball.
If you love PL games (i do), DI is one of 'em, and if you play it and learn it you will love it.
If you can't stand metal and/or GnR, flashing lights, and multiballs then GnR probably isn't for you. But it is unlike any other game that has ever been made. It is a one of a kind and amazing pinball experience.
GnR was made by the thousands, and is still available NIB. Once, they are sold out they will probably increase in value.
DI is limited, and will not be made again. But, despite its scarcity, DI will probably be available for the foreseeable future, (until they are all owned by PL/DI diehard fans.) Comparatively DI is a bargain at today's prices, and that isn't likely to change.