I always book "guarantees". As long as you are happy with an oceanview anywhere on the ship, you won't be disappointed. But, it has been my experience than when I made an individual booking as a "guarantee", I was given a substantial upgrade (that was not the case when I did so as part of a group, however).
I was on the Zenith 10 days ago, booked in a guarantee outside. We paid for a Cat. 8 room on a high deck with obstructed view, and got upgraded to a Cat. 6, unobstructed view on the bottom passenger deck. We wouldn't have minded either cabin category, but if you're at all picky about where your cabin is located (what if it's under the disco, or on a promenade deck with people walking past your window?)then a guarantee may not be for you.
Celebrity is a fine cruise line and I suspect you'll have a great time at a great price in an outside guarantee.