One thing about Royal Caribbean and Celebrity is that since they have started this program of upgrading older ships or building new ships with new features, not all sisters are created equal like they were in Celebrity a few years ago.
If you look at the differences between Solstice, Equinox, Eclipse, Silhouette and Reflection you see different restaurants.
For example Eclipse has Qsine but Equinox doesn't (it has Silk Harvest)...
Even more important, only Reflection & Silhouette have the Lawn Club grill and cafe. Those are very nice restaurants. But they don't have the glass show.
Reflection has far more Aqua-class cabins, but less room in "Blu" - the restaurant for those cabins.
So while all those ships are beautiful - there are differences one should look at.
Does it matter when you book? Not really. But as far as I know Celebrity no longer offers "price protection" once you make your final payment - you pay what you pay when the full price is due. If the price changes later you can ask for an upgrade or room credit, and I have heard of it being granted, but there is no guarantee it will be granted.