Horizon was designed in the late 1980s, came into service in 1990. There was no emphasis on balcony cabins, in fact, she has none. There's no internet room (though there may be an area now). The Horizon is pre Michael's Club, Martini Bar, Champagne Bar, Thellasotherapy pool, aft 2 level dining room with picture windows, aqua spa. However, in 1998, she received a Michael's Club, Martini/Champagne Bar, a very small aqua spa, but I believe no thellasotherpay pool. Horizon also had some of her hot tubs removed. What Horizon has, over Summit, is teak decks, and aft area for pax, not balconies. She also has an overlook outside of her observation deck for pax space. I regard Horizon as a midsize ship, not a small ship.
Summit was designed in the late 1990s, came into service as the 3rd in the series of the Millennium class. She has the specialty dining room, the big flower shop that Celebrity calls "conservatories", the 2-level listening room, and most of the features of the Century class except the aft-located lounge, a powerful thellasotherapy pool, aft space for passengers, teak decks, aft promenade that wraps around the ship. For her size she's lounge deficient, IMO. Summit is pretty fancy and fairly contemporary, but I've found that few experienced cruisers and Celebrity fans like her as much as they do the Horizon. I regard Summit as a megaship, but not one with a lot of public rooms.