disclaimer: I've only heard the Mahler, and can not comment on the Richard Strauss piece. I'm working from a friend's burnt copy.
Just flipping through Fulop's Mahler discography book, Sinopoli's Dresden Mahler 9 performance is among the slowest ever recorded (Levine/Philly is right up there too). Yet, it just doesn't sound that slow to me, thanks in no small part to some fairly creative tempo relationships from the conductor. Let me say first, that - in general - I'm no great lover of excessively slow performances of any music. For me, slower is seldom better. I wasn't easily snookered by Celibidache's last round of Bruckner performances, for example (some of it was quite good). But in this case, what greatly helps in making such an extreme performance work, is just the incredible commitment from the Dresden players - they're with Sinopoli every step of the way (and that includes fast parts as well as slow ones). Let's get down to specifics.
There are basically three different ways that the first movement can be dealt with. The most common is to clearly delineate between slow and fast sections (the climactic passages making up the bulk of the faster music). This usually makes the movement run somewhere between 26 to 28 minutes. But it also lends in making the first movement sound a bit more episodic than it probably should. The second solution is to take the slower music faster than usual. Abbado/BPO, Masur/N.Y., and Barenboim, are all excellent examples of this approach. These usually run somewhere between 25 and 26 minutes. This approach helps in taking some of the focus off the first movement, especially when coupled with a relatively slow finale.
The third approach is to do just the opposite: make the faster passages slower than normal. This approach treats the first movement as a large, single arch, and generally runs 30 minutes or more (Sinopoli is closer to 32). Although it's extreme, I find that this approach works quite well; such is the complexity of the harmonic progressions, as well as density of counterpoint, in this first movement. The one passage that sounds slightly uncomfortable with Sinopoli's consistently slower approach, is the string dominated one that Mahler marks, "Leidenschaftlich" (no other tempo indication is given!), and it's located after the collapse of the second climactic passage. But the main climax of the movement - anticlimax, really - is far stronger here than it is on Rattle's Berlin remake, for example. Again, the Dresdeners just play the pants off of this. Sinopoli's two inner movements work far better than you might suspect also.
In the second movement, Sinopoli is relatively fast with the initial Laendler. Thus, his first waltz episode sounds slower in comparison. Giulini did basically the same thing in his Chicago recording (DG), but Sinopoli does a better job of whipping up the tempo in the third and final waltz passage. Unfortunately, Sinopoli is also on the slow side for most of the autumnal sounding, linking passages. However, he and his Dresden players make the most of Mahler's many woodwind trills during these time-biding passages (that keeps the interest up!). The Rondo-Burlesque (third movement) starts faster than you might anticipate as well.
What's great about Sinpoli's R-B, is that you could set a stopwatch by it - the Dresdeners are with him every step of the way. Thus, instead of becoming monotonous - as is so often the case - Sinopoli's R-B actually makes one feel more excited as it goes. But in sharp contrast to that, Sinopoli is quite slow for the trumpet lead, central episode. However, it builds to a huge climax, with the Dresden horns belting out their melodic line at a full-throated fortissimo. The reappearance of the fast music is exciting all the way to end, with Sinopoli leaving room for a good accellerando for the last 20 seconds or so. The last movement is excellent.
Here, the Dresden strings play with just as much heft and amplitude as their more famous Berlin counterparts. Granted, they're being assisted by a far more generous, warm acoustic. The main climax is excellent, but Sinopoli does a particularly outstanding job with the final few - and very slow! - minutes.
In sum, this recording is probably not a first recommendation for most listeners. It's an extremely long performance, but one that's aided by tremendous concentration from the hugely underrated Staatskapelle Dresden. There are also a few moments when the sound quality is less than perfectly focused (as opposed to the players). But this recording may be for anyone who - like me - finds that the first movement can easily sound episodic; that the beginning of the symphony needs to be perfectly balanced; that the slow, central episode of the R-B should build to one heck of climax; that time itself, should all but come to a stop near the end of the symphony; or, anyone who's just a fan Profil's series of live performances from Dresden.