First let me say I am in the process of building a low rider since I want to be able to cut full sheets of plywood. However a much smaller application I’m considering is pub-style signs.
Setting aside the larder size that the low rider can do, what are the deciding factors to decide between the LR & MPCNC? What if I only wanted to cut up to 32"x24" (1/6th of a sheet of plywood)?
I’m not sure I understand the question.
Size is the largest factor. Both machines can do all the sizes, but a full sheet is far faster and more accurate on a LR, on the other hand a 10" squared build is far faster and more accurate on a MPCNC. The numbers start to be equal around the 48"+ total axis length (24"x24"…36"x12") above that LR, below that MPCNC. Introduction to The MPCNC - V1 Engineering Documentation
So for the 32" x 24" size, you would still recommend the LR?
Yes, that is 56" combined.
Just do what I did and build both 
I built both and haven’t touched the mpcnc since building the lr. The main issue I have with the lr is the shop footprint. You need to justify the space by feeding it jobs otherwise the big flat area turns into another table that gathers stuff.