Actually, the worst problems with NTSC occur when (non-broadcast) videotape is copied. DVDs are neither PAL nor NTSC - they're YCrCb. PAL and NTSC are analogue composite colour formats: nothing more, nothing less. (The two acronyms have nothing to do with the line/field rates of the underlying video systems, which are 525/60 in the US, Canada, Japan and a few other countries, and 625/50 everywhere else.)
A 625/50 DVD made by standards-converting a 525/60 disc master is *always* going to look worse, by definition (no pun intended!). You're throwing away 10 fields per second as well as having to invent around 100 scan lines per frame. Going the other way is no better, since 100 scan lines are lost and 10 fields have to be invented every second. It's always best to get video material in its original format if possible. (To see a *really* bad conversion, check out the French DVDs of Muppet Family Christmas and Emmet Otter!)
The Muppet Show was shot in the UK, and is 625/50 PAL. Most of the specials were shot in Canada or the US, and are 525/60 NTSC (just 525/60 for the B&W ones, of course
I don't know where Muppets Tonight was made.
Muppets Go To The Movies is 625/50 PAL, and it's possible that Tale Of The Bunny Picnic is (the MC timeline states that it was shot in London, which suggests UK cameras/video). All other specials - with the exception of VMC - are 525/60 NTSC. VMC was shot on 24p video, so the US DVD will be a 3:2 pulldown job, and the UK disc *should* be a 2:2 pulldown. I haven't seen the running times for the two releases yet, so I can't confirm that.
The Storyteller was made in London, so again, it should be 625/50 PAL, hence I will be waiting for UK DVD releases. Rocky Mountain Holiday, on the other hand, was shot in 525/60 NTSC, so I wouldn't be interested in a UK release should one happen.