I don't see the DGIV going anywhere any tiime soon. I voted that both are important, but I'll add a few specifics toward my complete opinion.
I love the XR2 for LEO/ISS ops. But, as has been said before, the DGIV is a workhorse, and it's my ship of choice for lunar trips and long haul flights. I'm taking one with me to the Jovian system on my current mission.
I must admit though that I am a bit biased toward the DGIV, since I was part of the beta test team. I've learned a lot from it, and its a great craft.
The XR series are also magnificent craft and are useful for a lot of missions (payload transfer with the XR2, station building with the XR5, etc.).
I use them equally, although I do hope a solution can be found to allow the DGIV to be used in the D3D9 client. I'd hate to have to switch back and forth to the inline client just to fly a DGIV mission.