I know there are some subtle differences between the 1 & 2, but I group them together since they're way more alike than different - especially in a mix. So I generally group the sounds together based on the original and new ("Contoured") blend. i.e. The MBD-1, MBD-2, and Bloop are one 'sound' as far as the blend goes, while the MBD-3, Fixer, and Mixer are another.
My layman's answer on which is best is distilled down to the following. Do you want some added low-mid/mid punch? Or do you want more subs/lows with unaffected mids? The former is the 'classic' blend pedals, and the latter is the 'contoured' blend pedals. Sure, there is more to it when you consider feature sets, etc. But I think figuring out your blend preference is a key starting point, because they feel and sound different in a mix.
I have 2 Bloops, 2 MBD-2s, an MBD-3, and a Dirt Fixer. I have some uses for the latter two, but after trialing the contoured blend I prefer the classic blend for my sound. So the Bloop and MBD-2 are on my board.