I second Dave's opinion. I like them both, and they're different enough to justify owning both. Unfortunately, the mezzo on the DG one isn't particularly good. But there are many other fine things about it. Personally, I like it much more than the Boulez/DG one that also employs the Vienna Phil. (which has NO organ).
The organ isn't that bad on Kaplan's DG one at all. I'm sure that this was at Mr. Kaplan's insistence (based upon Mahler's fortissimo marking), but DG dubbed the organ in from the Berlin Philharmonie. My understanding is that the organ in the Musikverein - generally very weak for 'big' works - has since been refurbished.