i have found that elites are a lot tougher than spartans, they can take a few more shots when they have no shields. The female character model may be smaller but the hitboxes are exactly the same as the males, so while your head may not be visible, the hit box is just above what your taking cover behind. so you can still be headshotted.
I'd say the Elites are my fav simply because they have evade which is such a usefull tool. I don't fully understand why sprint and evade are armour abilities though, they should just be standard.