hey, I would def recommend balance druid for several reasons:
1) battle res (very important in mythic +)
2) trade loot with resto/guardian druid/DH (you will see a lot of them in mythic+)
3) decent AOE for trash mobs
4) stealth (for skipping trash mobs, without invisi potion)
5) druid specs are strong overall in mythic + (resto/guardian), so you can switch spec
i've never played shadow priest in beta but i've heard they are not ideal for mythic + runs, as well as retribution paladins and warlocks
Shadow does pretty good damage honestly, a lot of trash packs are 3-4 and that's basically the sweet spot for keeping dots up on everything. I realize that they may seem like shit because in most instances trash dies really quick anyway, but if trash is dying really quick anyways it doesn't matter what your shadow priest is doing, it also means that you probably won't have a problem with that difficulty. As the difficulty rises the health rises, meaning that contribution wise you'll see what shadow priests can actually do if their dots last a full or couple rotations.
VE can heal a lot, they can bring an AoE stun with mind bomb, and they do an amazing amount of damage with StM which I assume will be used even in Mythic+ as you can likely use it on several bosses.
Please use this thread to discuss your choices: http://www.mmo-champion.com/threads/...ses-for-mythic It already has some decent discussion on some of the classes and maybe you will get a better idea what might suit you from there.