Frost, since you don't have many stats for Arcane too (plus mastering movement for it is annoying for a Mage looking to get their feet wet).
Frost will also help immensely in your Heroic Dungeon grinding (should you choose to do them to get gear for LFR iLevel reqs) by using Frost Bomb, then Frost Nova or Pet Freeze before it explodes to Shatter it
Frost bomb would be better than Living bomb?
And I should still keep Invocation and Presence of mind?
Frost Bomb is immensely better in DUNGEONS, which, I'm assuming you'll do first to gear up for the iLevel requirement to do LFR. Once you get out of 5-mans, swap to Living Bomb (Better for ST) or Nether Tempest (Better for a few targets stacked close). LB is overall better though for ToT, and I'm not sure about Siege as I haven't seen/read a single fight yet
Invocation is by a good margin, the best L90 talent for Frost (and Fire), but you could use IW if you'd like to be lazy/get a hang of the other things to worry about. Don't use it (IW) in any serious raid, but feel free to use it for Dungeons/LFR because, I mean, 9% damage isn't the end of the world in non-progression raids
Your L15 talent is ENTIRELY up to you. PoM isn't all that helpful for Frost, and Ice Floes just got a massive buff today, so pick either Blazing Speed (my personal favorite) or Ice Floes.
I dinged about 3 weeks ago and in my experience arcane is perfect to play IF you enjoy the spec and can plan ahead (for proper Rune use). The damage, especially in dungeons where you can stand still and just burn through your rotation without moving, is TOO DAMN HIGH, single target as well as AOE.
I would go Frost. With respect ot the level 90 talents, Invocation requires the least amount of maintenance. I find Incanter's Ward really annoying to try to use effectively. Rune of Power is really only good for Raid Boss encounters, you're going to be moving to much in Dungeons. Just set up some kind of weak aura that reminds you to hit Evocation every minute and you're good to go.