Help Me Decide on First Salt Tank

First post here, my names cody, i live in East Tennessee, im a mechanical engineering student at UT. I have had 11 freshwater tanks over the course of my life, but until moving to knoxville i have never had access to saltwater supplies. Now that i do, im obviously excited!! My thinking is to start with a FOWLR to acclimate myself to the added variables of saltwater. Ideally moving onto a full blown reef setup once my experience allows.
My question stems from the age old discussion of the law of diminishing returns....
I already have a 40 Gallon breeder tank, heaters and powerheads.
I also could easily buy a 20 gallon breeder for little money. Those are my two options regarding the size of the tank. The sump filter, lights, stand i intend to build, as a fabricator im rather savvy at DIY. The question is, is the larger start up cost (for live rock at 7$/lbs) for the 40 gallon worth the extra size as opposed to the obviously lesser cost of starting a 20 gallon. The cost of live rock and substrate is my main concern here.
In summery, is having a 40 gallon breeder over a 20 gallon breeder worth the extra start up cost and running maintenance when it comes to long term enjoyment, ie the number of fish available, stability, etc.

Feel free to point out any other variables i have left out regarding the initial costs that may help, hurt, or render the comparison useless!

Thank you for your time,

Cody