I'll chime in for the 18 strength warlord. I'm playing an inspiring dragonborn warlord with 18 strength and 16 charisma. I can't imagine playing him with those stats reversed. I think 18's in both would also be good, but I like having a 14 Con (for the extra HP's on healing surges and future Combat Veteran) and 13 Dex (soon to be 14, and later 15 for scale specialization, evasion, and HBO). I even plan to spread out my stat increases, my strength will go up every opportunity, but my charisma will go up every other opportunity.
Unlike certain other classes, I think the inspiring warlord can actually get away with a lower than maxed out charisma. One less charisma typically just means 1 less temporary hit point, or 1 less damage, not a huge deal. Hitting on the other hand is always important, especially for encounter powers.