Just as Fandreith pointed out, M/R imho is very fun to play, and it does pretty good damage in both - PvE and PvP. I have one of suggested M/K combo, but M/R in my opinion is much better.
First, as mage you're always glass cannon, you will produce a lot of damage in short time every 5 min for 20 seconds. Elemental Catalysis + Ancient Spirit Water + Shadow Protection (or Clear Though) + Curse Weapon Rune and Pet's buff and you will produce a lot of damage in short time, and if tank can't hold aggro, you better stack a lot of stamina and HP.

As we're limited to 2H staff (better then any dual weapons), we can have only 1 recon rune, thus even if you produce less damage from OP class such as R/S, you will get aggro more likely as they can have 3 recon runes.
What makes M/R great is Cursed Fangs + Fang Ritual. To play as M/R you have to have Fang Ritual always running (ntBuff does the job). It's almost as having Fireball without bad cool down and with additional DoT (Damage over Time). So you can cast Flame, Fireball and spam CF until mob is dead. As CF is projectile cast, you can cast it while running. You share 2 great AoE every mage has - purgatory fire and thunderstorm. First one is great for close fight, where other one is great as cast on distance. Mostly, tank will aggro group of mobs, you will cast Thunderstorm and any mobs that survive and gets close to you, you clear with purgatory fire.
Kiss of the Vampire has bad cool down, but it is nice spell that returns 10% of HP. I don't use it in my rotation as I can't spare too many TP on it. At higher level it does increase damage of your next 2 magical attacks as well, thus it is good, but as we are always short on TP, you might hold on maxing it until you got other spells maxed.
R/M are popular in SW as they can lay down traps even if all structures in Castle get destroyed. I was surprised at amount of R/M in my old guild, just at the time of SW.
And just to mention M/K combo, it does give a bit more HP and defense, but you're missing those good instant casts and light magic is not as fun as dark magic.
As this is mostly gear game, almost all combos are doable, but some are more fun to play then others.
Hope this helps