Each side is picked to attack ('speed') based on the ratio of their total
attack value: 1 noble/25 peasants vs 1 noble/5 soldier would have...
105 vs 105, which doesn't sound that unbalancing.
This does couple the attack rating to the rate of attack, however,
which might not be desirable; for this model, it might be better to
have the 'speed' factor which initiated this discussion.