Welcome!
IMO if your very first cruise was on a Solstice class, you haven't really cruised.
"New and fresh" do not make a good ship. Regardless of age, ships are updated and refurbished, which you SHOULD pay attention to before you sail. The Waldorf Astoria in New York is very old too, but one of the finest hotels in the country. Look at the Century, which was turned into a completely new ship recently.
The Century class for me, is a bit small, but many many people love that class.
Unless you are geared for a "playground" like atmosphere, or really like having 10 restaurants to choose from, you should definitely try Summit. The M class is spacious, sophisticated, and offer more than enough diversions. The cabins are actually better than the newer ships as they spent more money on things back then, like wall thickness and structural issues.
The Solstice class does not offer a true ocean voyage experience IMO. It's more of a enormous floating high rise on the ocean. I sailed on a ship a bit smaller than her (The Princess Ruby) a year ago and couldn't even HEAR the ocean from my cabin, it was so far down.
Go to celebrity.com and take a virtual tour of the different classes of ships and amenities. That should give you a good idea which class is best for you.
