I have a variety of disks on SACD (stereo and multi-channel and even mono). I also have a variety of DVD-A disks including pure DVD-A and DualDisk. Frankly, I have to admit that I've never used the menu option on the DVD-A to select the stereo mix, and I've never tried playing the 5.1 mixes in stereo. I guess I never have because I bought the 5.1 mixes to listen to the music in 5.1. I already have the music in a variety of other stereo formats.
My impressions of the formats:
SACD (mono) - I forget who the artist is, but she's a jazz singer from the 50s. The sound is good, but clearly the greatest limitation is the 50+ year old source material. The recording technology just wasn't as good then. Still, there's some real nice sounding stuff.
SACD (Stereo) - Two disks, both by Mobile Fidelity Sound Labs. The first, Los Lobos' Good Morning Azatlan is awesome. Close your eyes, your system disappears, and there's a band in front of you. Great dynamics. Great sound. The other, Edgar Winter Group's They Only Come Out at Night... I just got this one, and it sounds a lot flatter than the Wolves. But, then again, it was originally recorded in 1972. I mean, it sounds good, but not that good.
SACD (5.1) - David Bowie's Ziggy Stardust and PF's DSotM - both sound really good, but I'd definitely give the edge to PF. Bowie's is good, but it just doesn't hit you with that musical presence.
DualDisk - Contrary to other posters', I've had great luck with mine. Most of mine are in the Talking Heads' Brick collection (all 8 of their studio releases). Jerry Harrison played a role in re-mastering the stuff, and it just knocks my socks off every time I listen to it. Especially the entire album of Speaking In Tongues. The dynamics are so great, the music is tactile. This is what hi-res digital audio should be like all the time.
Pure DVD-A (various mixes, but most 5.1) - These are also very, very good. I just got Steely Dan's Gaucho, and this, along with the Talking Heads' Speaking in Tongues are probably the two best sounding DVD-As I have. Oh, and Donald Fagen's Morph the Cat. Just crazy good how good these suckers sound. I put these disks on, and I just sit there smiling like an idiot it sounds soooooooo good. To me, that's what this hobby's all about.
I have lots of others, but these are the most notable in my collection.