If you really need to be on that ship at that time, then I would deposit the best cabin you can and watch the pricing and availibility. Or if another ship will do, deposit that and watch the pricing on the Summit. If it does not fall before final payment you have a decision to make , I guess. Although I understand Celebrity is still good to deal with if the price drops prior to sailing, after final payment.
The above posters are right, cabins will likely become available closer to sail date, and you can know by checking sites litke travelocity and icruise.com which show the available cabins in any category.
Did you call Celebrity to find out it there is a large group that has booked the ship?Some groups block out 100's of cabins and then what doesn't sell is released.
Personally I don't think any ship out their is worth paying twice the regular price for.
We wil have been on 12 cruises by the time we sail on the Mercury in February for 11 days. The price was very right , so I booked it , but went inside so we can recover fromt he financial shock of our 17 night HAL cruise in a CAT A Balcony ,November.
Good luck with whatever you decide.