John,
Get the Jansons/RCO M6th by all means. It may not shake you on your first hearing but don't give up, as Mahler's music often comes into life in different moods and directions. You'll admire and love the playing of the Royal Concertgebouw Orch.; it's pretty much their show despite Janson's decent conducting.
If you love the Jansons then I'd recommend Ichiyoshi Inoue/New Japan Phil./Exton recording. It may be currently OOP but with a little luck you might be able to get one copy somewhere. IMO, this one the most prefect musical structure of this great piece. With Scherzo coming in II, each movt. is beautifully proportioned in terms of small sections that connect each other to make the whole, and the entire symphony as a whole holds as logically and aesthetically well as I can imagine. Except for the Levi, I know of no other version that gives me such a sense of uniformity and logicality. Be warned, however, there is a minor mistake in timpani in IV. in the development section. I won't go into the detail because I don't want to spoil your listening experience.
John,