Well, I certainly won't interfere with the great info. that Scott has given you. He and I part ways, somewhat, on the MTT/SFSO cycle in general. Perhaps it is a better recommendation for the 4th than the Boulez. But I like for the 4th to be on the swifter side - more "neo-classical", I reckon. As I said, it's a pitty that UNI hasn't issued the fine Chailly 4th in an SACD.
What little I've heard of the MTT 5th was a big turn-off to me. Then again, I would certainly I agree with Scott's contention that any M5 should have a great finale. I just simply didn't get that far with MTT. If the Markus Stenz M5 were available on an SACD (ABC Classics), I'd say to definitely get that one. However, even the regular CD version of that is expensive, and has to be shipped from Austrailia. Stenz is now the conductor of the Gurzenich Orchestra in Cologne, which is the band that gave the world premiere in 1905. As you might imagine, Stenz takes the 5th very seriously. His recording is great from start to finish.
The 8th is one case where I don't think that MTT can manage to screw it up, even for me. I've seen him do it twice, and he doesn't do any sudden, weird slow-downs in Part 1 - one of his annoying, pantented trademarks throughout his Mahler. Also, Davies Hall will be excellent for recording the 8th. There's tons of room for large choral forces, and the organ is louder than all get-up. It should be good, unless he gets even weirder and more self-important in the mean time.