Starting with the latest: reference 6656, the new 40mm version in steel, rose gold or platinum, with base calibre 1151 and either a plain white dial for steel or opaline and blue sunburst dials for the precious metals cases.
Reference 6057 is as you already mentioned in the line up for quite a while, it only comes in a 38mm rose gold case. The base calibre here is also the 1151.
Reference 6659 is the larger 42mm version, introduced around 2013/14, in either rose gold or platinum with white Grand Feu enamel dials. This one has calibre 1315 as a base, enhanced with titanium balance and 8 days power reserve. Generally the 8 day calibres receive more hand finishing than the 1151 based 4 day calibres.
With no steel QP in the offering since the 40mm Leman Quantieme Perpetuel was discontinued some 4 years ago, Blancpain wanted to not leave this space uncovered and launched the 6656 exactly one year ago. In comparison to reference 6057 the new Villeret dial layout was chosen, and 40mm seemed to be a universal size for a timeless dress watch.
The 42-45mm Villeret models (there are quite a few like the Demi-Fuseau Horaire, Pulsometre Chronographe, Quantieme Complet, Equation du Temps Marchante,...) with Grand Feu dials (black or white) are considered one step above the „standard“ Villeret watches, offering more in terms of decoration, complexity and savior faire.
Hope this helps to navigate between the options.
Cheers
Henrik