For garrison duty, I think the size of the unit matters when determining loyalty effects so equites may not be the best. However, I agree if you think you are in danger of having to use your garrison to fight... In MTW, I tended to use reasonable troops for garrisons, but in RTW I have not found it important yet and rely on Town Watch.

I agree with the general point that up keep costs don't seem well thought out. Basternae mercenaries are currently my favorite undercosted unit - 2HPs and very powerful attack, but have the same cost as Hastati.