Well, at street prices for 512cs and a lunchbox, you essentially get a 4th channel free by buying the 3124. And used 3124s sometimes go for less than $2000.
It appears as though the lunchbox set-up offers the most flexibility; since (or if ) the channel economy aspect of the API 3124+ is not so much a factor.
A rational choice: Lunchbox, stock it up with a myriad of "flavors", apparently there are several available mic preamps in that format (including, but limited to: Great River and API). The option to add additional (flavors) pres as needed or when possible at a reasonable cost per channel.