I assume you're talking about some mod here Fyron... in stock Monolith III produces 700 of each resource per turn, and as I often get into slight economic trouble early on (nervous laugh) when my Min stores hit rock bottom and I'm forced to put all but one queue on hold, I prefer to go with the 1k Mineral Miner III (or am I mistaken here) to produce more resources... also getting hi-lev Miners/Farms/Extractors requires less research than getting Monoliths.
(And btw - I never build Monoliths on gas giants. Everytime I do, they seem to multiply and turn the planet into a small star system
)