FYI: Set A release is commonly known as "Animeigo" while Set B is known as "ADV".
I believe the consensus here is that, unless you really prefer dubs (no matter how badly done) over reading subs, then Animeigo is the way to go. Animeigo's sound quality is superior to ADV's and the whole DVD set is cheaper to boot!
To answer your questions:
About the animation (or video quality): Both use the same restored video, but Animeigo has the edge as it is progressive scan whereas ADV is interlaced
About the subtitles: I believe both versions use basically the same subtitles with some minor corrections in ADV. They are official subs anyway so they are as good as they get.
About the dub: I think one member of this board described ADV's dub as "beyond suck" and many others have voiced similar opinions but this is a subjective matter. Personally, I agree with those comments, after previewing some episodes I felt some characters were overacted, some accents were way too "American" for my taste (nothing personal, mind you), Hikaru's voice was childish/unbearable, etc. Also worth mentioning is the fact that ADV's English track has had some (all?) sound effects replaced by different versions.
For more information you should check this nice review written by Hurin and also some past threads here and there (there's some confusion in the first 30 posts of this one, though)
The only complain about Animeigo's release was regarding the discs' cracking tendency (something to do with the material). Not sure if this problem has been addressed already.