Shawn,
From the parts I listened to in the 2013 Gielen recording it doesn't seem like there is much contrast. The entire movement is just consistently slow, which is how he's able to achieve a duration of nearly 28 minutes! I think Chailly pulls off the slow tempo just barely, but it's constantly encroaching the brink of being too slow. This is just my opinion, however. I actually do like Chailly's Decca recording. I believe the only place in the first movement that should be notably slower is the soft "oasis" section of the development. Currentzis does that exceptionally well in his recent recording (a release that I found to be well-played but awfully recorded).
If you like slow tempi with intelligent contrast you may like Osmo Vänskä's recent recording with the Minnesotans. His march tempi are quite slow (like Barbirolli, Chailly, Boulez, etc.), but his Alma themes just a bit more urgent. He also zips through the coda, which manages to work if you switch the inner movements so the scherzo is second (as Barry has said before, and which I wholly agree with).
For the most part I agree with your comment that Bernstein's DG Sixth is excellently paced and crafted, even if on the slower side of the norm. I find that recording to be one of Bernstein's best Mahler readings on disc. When I think of Bernstein being to slow his DG accounts of the Second and Third come to mind. Overall I find his DG M3 to be superb, with a slightly too slow "Mitternachtslied" and far too broad a finale. He wallows a bit much, to me. I prefer that movement around 21–23 minutes—like the Fischer brothers and Chailly—instead of 28 minutes. His Second with DG is just too slow for me, but I understand the appeal; it's very movingly performed and conducted. Like you said, slowness is relative and I definitely agree with that. The great thing about Mahler's music is that it's all about how everything fits together as a whole, not just a particular moment being "too slow" or "too fast."
Erik