Well from my experience, Blu-ray and DVD quality does not differ too much on visual and sound quality. Even with sensitive ears like mine, the difference in sounds is very small.
There is a HUGE difference between Blu-ray and DVD at least visually. I watch it on a 42" screen so if I put on a DVD it looks ridiculous compared to the Blu-ray. Some of Momoclo Blu-rays (the ones where they split one concert in two Blu-rays, like for example Momokuri 2012) look outstanding, even better on HD than some Hollywood movies I've seen.
As far as sound goes I don't know, I guess DVDs also come with 5.1 but I don't know if I would be able to tell the difference.
Yeah the visual difference is massive, and the reason Momoiro Christmas 2012 and Peach for the Stars both look so good even compared to other releases is because they have extremely high bit rate encodes thanks to more data storage being available across 2 discs rather than 1. I greatly appreciate this, sadly many companies are willing to sacrifice the video quality to save money instead of printing extra discs.
As for audio, bluray releases from Yokohama Arena 2 days and onwards (some exceptions) all have uncompressed soundtracks in both 5.1 DTS HD surround and stereo sound. DVD releases have the stereo soundtrack only. The older stuff that was released on dvd first and got the bluray treatment later on missed out on surround as has the Kodomo Matsuri releases where it would be pointless. If you do not have the set-up then I suppose this doesn't matter but 5.1 is very nice if you can get the chance to hear it and certainly not a small difference.
I didn't know the DVDs only got stereo, that is kinda sad. It certainly adds to the experience when you can hear the mononofu chants surrounding you. I'm so used to it by now that when I watch some of their performances on festivals or something that only has 2.0 is like I've gone half deaf, like something is missing.
Blu Ray is definitely the way to go if that's an option, sadly some artists still only release on dvd (I'm looking at you Dempagumi.inc)
I just finished setting up a 100" 1080p projector setup in my living room. It is AWESOME! I watched the Summer Dive 2012 concert on it last night, the panning crowd shots made me dizzy in a good way. The projector can handle 3d as well, so my question is, will we ever see a 3d Blu Ray release from Momoclo? \m/
Something interesting Momoiro Clover did a promotion for Mitsubishi 3D TV's where they filmed a special 3D PV for Kono no Uta. It sells for around $300+ on yahoo auction and there is only 1000 copies based on this info http://natalie.mu/music/news/48681. Really would love to see it and yeah it sucks when artists release on DVD, hurts even more when it was obviously filmed in HD based on promotional clips and TV concert excerpts.