Actually I would not recommend the C cabins personally on any deck. There is a vibration at the back and the angle of the waves, speed of the ship can affect all those aft cabins.

That being said those SEven Seas Aft suites are fantastic so I would probably take one personally because the vibration would not concern me. I LOVE the layout of this cabin have not sailed in one yet but would like to do so. The plasma TV and the floor to ceiling window are really great.

However, with the C cabins since the cabin is the same and just a bigger balcony I would suggest a more forward cabin. I dont think the larger balcony is worth the risk of vibration.