I've never touched the medium looping bag, but the Manhattan PM definitely will hold less than the Speedy 25. The Manhattan is a lot thinner and doesn't go as deep. I would still go for it though. I really love my Manhattan. You should definitely go see it in the store. It's a beautiful bag.
Run, don't walk to get this bag. Can I tell you how much I this bag!?!? I love everything about it! It holds a decent amount of stuff for me but I don't carry around my whole life in it either but it holds its fair share. It's comfortable to wear and not that heavy. The upside is that since it has a canvas bottom, you can set it down and not have to worry about getting it dirty too! That's one think I don't like about the Cabas Piano (vachetta bottom). Anyway, I'd say go for it, you won't regret it!
PS - I got the last one when I bought mine....guess they sell out quickly but I haven't seen too many being carried which is a good thing.
I my p.m. And I really like the fact that, like jasanna143 said, it's not that common. It's roomy without being oversized, and it's structured without being too rigid. Love the pockets too.
But keep in mind it's strictly a handheld bag (unless you are tiny) and it's a thinner bag. Are you okay with big (semi flashy) hardware? I think that's another thing that keeps some away from this bag.
One tip, some of the SAs I spoke with tried to convince me that the Manhattan bags are only made in the US. NOT TRUE. If you care about it being a French-made bag (just my personal preference), try calling the LV boutiques and dept. stores in Vegas if you can't find it in your area.
FYI (from the LV site): 11" x 8.3" x 5.5" for the Manhattan PM