There is something to be said for having a complete cycle with one conductor and one orchestra. This Jansons one is probably very consistent. I think I'm more inclined to recommend the 'maverick' Gabriel Feltz cycle, or the really well recorded Jonathan Nott one. I remember, John, that you were 'big' on the J. Nott M9. Frankly, I would take that one over Jansons M9 any day. If Oehms hadn't screwed up the sound on the box set issue of the Markus Stenz cycle, that one WOULD have been highly competitive. Oehms down-graded the sound quality, for some reason.
There's so much good Mahler these days. I think the Warner edition of the Tennstedt box is pretty competitive - the one where they included both the studio and 'live' recordings (not to be confused with the live recordings the London Phil. issued themselves).
I'm also a fan of using one orchestra, but with a number of different conductors. I do like that Berlin Phil. box. I wish the Vienna Phil. would do the same thing, but they would probably screw up the curation of the thing.