Basically the MK views from the Poly only allow you to see the tops of the castle and Space Mountain. If you really want an in your face view, the Contemporary is the only way to go. That said, the Poly MK views also look out on the beautiful 7 Seas Lagoon and have a dead-on view of Wishes. Also, if you get an MK view room in Tahiti, you get the bigger rooms.
With the GV Concierge, you lose the beach view and the larger room, but you get a nice view of the quiet pool and the pathways leading up to the Hawaii building. Hawaii also has a more central location in the resort. You also gain access to the lounge with IPO services and free continental breakfast (which is a really nice perk) and snacks and beverages throughout the day. The other nice thing about the lounge is you can go there and have probably the best view available at the Poly with its two story floor to celing windows. They also pipe in the soundtrack for Wishes. The lagoon view might not be right outside your room, but it is close and for the same price you get some other nice perks too.