The cost difference is almost negligible. The bigger concern is durability vs synergy here.
Rat Colony grows even with
Marrow-Gnawer's tokens but gets wiped out by any cheap 1 damage to each type effect,
Blazing Volley,
Marrow Shards,
Sandstorm,
Dry Spell, etc., if you're not including ways to increase toughness
Ascendant Evincar and
Death Pit Offering are the ones that immediately come to mind for mono-Black. Even if your group doesn't normally run these types of spells, if the deck becomes a frequent thing they may choose to add some in if you can't reliably bulk the rats up.
Relentless Rats has a cap on how large they can make themselves due to only caring about
Relentless Rats, barring something like
Helm of the Host being in the deck, but they're going to require actual board wipes to get rid of.
Personally, I think I'd go with the
Relentless Rats outside of possible budget concerns since they run almost an extra dollar each.