I have been favoring the paladin, for the slightly speedier leveling.
Does anyone bother saving up for the -1 stack for 45 points? Its a great ability, and -2 would be amazing...but 90 points seems out of reach.
A system whereby the price of an ability increases with each purchase would promote more balanced heroes. So, your first purchase of command costs 20, your second 40 and so on. This would also serve to differentiate the heroes further, because bringing an initially low stat to a high level would be quite expensive.