This has been discussed.
Note I disagree with some authors, even my older posts, as the game keeps changing and different rosters are improved with different strategies.
Link
(Farming and TC strategies: Backpacks vs Rugged Clothes vs Recruits (Grab Bag))
(Tima's "Unofficial" Official Guide to Speed Levelling)
(ARCHIVED Before Advanced House 9 and Atlantis Rising, Uncommon training versus Extra fast training)