to be fair 7% extra damage from garania to 591 damage that aw mehlis has at level 80 is somewhere around 42 damage. Max level awakened Marius does 683 , so pure attack damage , he outdamages her even with or without garania's boost. The splash radius is the same , and as a bishop he has 10 base range more then mages , and his awakening gives him 15 more. That means he has a permanent 25 range over mehlis when mehlis doesn't uses skill
As an elf he also attacks faster then normal mages , but that's countered by the fact that mehlis also gets attack speed increase after AW.
This is how i see it : if you want a mage that can hit at a very far range for a couple of seconds , but hit for less for each hit , u go with mehlis. If you want a mage that will hit at a longer permanent range , but lower range during skill (well his skill actually heals , so it's not even a mage skill) , you go with marius.
Personally i think marius is better in terms of both dps , and versatility , but he;s also a plat so that means he's a bigger investition.That , and i know that mehlis gets a very very good Skill awakening eventually.....but untill then , he is prob going to have both maxed. In the end it depends on himself. Both are good mages to have anyway , so he can't go wrong