Go into a shop & try both on, see if the armour feels right in them, you want it just cupping around your elbow. shoulder armour is more adjustable, but different brands may have different arm lengths. try it on with your gloves too, to make sure you have a full range of motion.
Mesh trades abrasion resistance for lightness & airflow, I personally wouldn't consider it (but I don't live in a very hot climate)
This is what I wear