Bamspeedy
CheeseBob
hbdragon88 - you must mean not to build warriors. Spearman upgrade all the way to mech infantry. Warriors only upgrade to swordsman.
Building catapults and spearman early on for later upgrades may make sense. It depends if you want to be devoting your shields for that purpose that early in the game. Sure, they cost less to produce, but your civ also has few shields to spare! And, like you mentioned about upkeep costs. Depending on what level you play you could use that gold to set science high and get a tech lead, so you wouldn't be too pressed for having a massive updated army right away.
Building catapults and spearman early on for later upgrades may make sense. It depends if you want to be devoting your shields for that purpose that early in the game. Sure, they cost less to produce, but your civ also has few shields to spare! And, like you mentioned about upkeep costs. Depending on what level you play you could use that gold to set science high and get a tech lead, so you wouldn't be too pressed for having a massive updated army right away.