[bbz]

Dont give you mages many gems at one time since in battle they tend to use more gems than it is actually required for casting spells, so that they can reduce the fatigue of the spell(buy gaining 1 magic level per gem used). So what you can do (to reduce the ammount of gems you spent with mages) is just give most of the gems you need to scouts and move them with your armies only giving to your mages the minimum ammount of gems they need to cast certain spell.
About what is better I havent got much of experience myself but I'd assume as you said depends on the situation. If you get them an item (winged shoes, crystal shield) that is always going to be useful. But remember that some spells might turn the tide of battle(earthquake, darkness, master enslave) so I think you need both, but gems beaing really important when used right.