I have both as I am totally in the CD world...and will add my comments here on my opinions, information, and readings...
Most seem to like the DSD versions over the PCM versions. Yet I do have a few PCM versions that are very nice to listen to, so it is not the bottom line to a good SACD recording.
There are some SACDs that sound better in MCH than 2Ch
There are some SACD's that are mixed way too aggressively in the surround channels and make it sound like you are sitting in the middle of the band instead of watching them. I prefer the MCH SACD's that help add the ambience and depth to the recordings, not make the performers sound like they surround you. In other words, you do not think the surround speakers are being used.
Some of the classical SACDs, like the RCA's are three channel as the orginals were done with three mics. While these were done in the 50's, they are very nice recordinds. Like anything else, if the orginal recording was done well, a re-release in SACD will sound good too. You cannot make Chicken Salad from Chicken Poop...as the old saying goes.
The only redbook and SACD that I have both of is, Hourglass by James Taylor, and while the Redbook is a good Redbook CD, the SACD is slightly better. But again as I stated before, if the orginal was bad, so will be any re-release, remaster, SACD, etc. For instance I have the Dire Straits - Brothers In Arms on Redbook, and remastered Redbook - the remaster is slightly better, but it is still rather "CD" sounding. A new SACD of BIA is out now and I do not expect it to be much better than the remaster. Yet I have a redbook version of "On every street" by Dire and it is a very good sounding Redbook CD - it walks all over BIA by leaps and bounds.
Playback of SACD is only as good as the player as with Redbook. This is really the key to good SACD playback. A bunch of people are having mods done to SACD players to get even better playback from them. These mods can run from a couple of hundred to a couple of thousand dollars depending on the mods. There are also some real rip-offs going on in the mod world for SACD - you just have to look around, see what people are having done, and who is getting the most positive talk.
Most of the good SACD's are either Classical or Jazz. Most of the Rock SACD's are just not that good. I do have the John Mayer SACD and it is one of the better rock SACD's that I have - which is not many. But there are rock SACD's out there if you like the music and just have to own it.
Prices for SACD's run from $10 - $20 on the average from what I have seen.
If you are interested in trying out SACD, get a player from someplace like Tweeter, bring it home and listen to a couple of SACD's. A player that has received some nice reviews for a name brand, main stream player that can play redbook and SACD is the Pioneer DV-59Avi. Some of the newer players getting good reviews is the Marantz DV-9500 and Onkyo SP-1000. All of these are Universals.
Frequency response - Never really checked on it to see.
Most of the SACD's that I listen to have a better all around sound than most Redbook CDs. But you can find good and bad Redbook and good and bad SACD's.
Hmmm...did I forget anything....not sure...but I am sure others will jump in here with their ideas and opinions. Like anything else in the audio world, you just have to listen for yourself and form an opinion.