i would either get that mage to 99 or get that strength up

I say get the 99 mage it would benefit you in the long run.

Unless he's planning on Dunging 99 will not benefit him any more than 96 will in the long run it will just give him a not so fancy cape and Archmage title if he gets 99 Runecrafting for the title, but in terms of power you're no better at 99 than you are at 96 outside of Dungeoneering,of course though there's the exception of Celestial only in Dung.


Edit - however Prayer is useful if you have no intention of getting curses you could still at least get 45 for the f2p prayers which are useful and if you're p2p might as well get 70 for all p2p prayers (excluding curses)

If i had that account and still played runescape id grind for 99 mage

Well it's worth having at least one 99 I think I've worn my Magic Cape once since I got it many years ago, I don't like the cape although it's my main attack style.