On the Grand class there appears to be a balcony accessible from the hallway on the Riviera, Aloha, Baja, Caribe, Dolphin, and Emerald decks, aft. At least it looks that way on the deck plans. Does anyone know if those are sun decks with chairs, or what exactly that is?
A cabin located aboard the ship is definitely preferable to a cabin located on the pier.
Beyond that, the location of a cabin really does not matter too much. The cabins nearest amidships theoretically experience less motion than those nearest the bow and the stern, but the reality is that modern cruise ships usually do not have noticeable motion in the first place, thanks to the combination of their size and the use of dynamic stabilizers to counter the ocean's torques. I generally prefer a cabin located above the Promenade Deck because it has more convenient access to the topside facilities (pool and buffet areas, etc.), but the cabins below the promenade deck don't have balconies so even that is not a consideration for you.