For me, the Six series fall into a "not good for anything" category. That is only because I have other .357's though. For a HD or range gun, I want the weight and heft of a GP100(Or in my case, a 686 Competitor). For carry I want something smaller, more easily concealed than a six series gun, and my SP101 fills that role.
If I feel like carrying a full size .357, I carry my 3" 686+. I like the lines a little better than the Six series guns or the GP100, and I get 7 rounds.
Personally, I think the GP100, the Six series guns, and the SP101 are all great. But I have my needs filled by my SP and 686's. The Service and Speed Six guns would accomplish nothing for me that I can't already do.
Just my scenario though. For a new buyer, the Six series guns have appeal. I think that's part of why S&W re-introduced the M66. they saw a bit of a void that they hope to fill. It isn't a huge void though IMO.
I like the more solid cylinder ejector rod on the Six series better.
Say it again. The ejector rod is the one thing on both of my SP101's (same as the GP's I believe) that seems really lame. I haven't had a problem with either, but they just seem wobbly and poorly conceaved.