There is no real system, it really depends on you. How do you want to fight. For instance, you are the Britons, you pick up some Woad Warriors, incredible attack, incredibly awful defense. You have enough for three weapon upgrades each. You can try to increase your defense, so they last longer, or you can try and up the attack so they tear through enemies with ease. Well, how will you be using them? As your main battle line, or as shock troops?
A few tips, defense upgrades, whether by experience or by shields, do not improve defense against arrows, or any missiles. Shields help when attacked from behind, while experience helps when fighting head on. Shields do that as well, but experience also ups attack and morale. So to be honest, shields are not that useful, unless you want to surprise someone with the stoutness of your line and you don't have the cash for chevrons.
Also, you usually don't want to half ass swords or shields. Go all the way, because plus one isn't that effective, whereas plus three, with no obvious changes to the unit, can be surprising. Your opponent might not expect some units to be so powerful. Experience makes more sense to distribute. Your whole line is less likely to fail.
Share our wealth!Nobody expects the Spanish Inquisition! As far as I can tell, Baltimore is in redneck country ~ Bored Scotsman You all realize that both Halo and Starcraft stole their storyline from tetris, right? ~He113ent