You have had some excellent advice here and I’d agree with it. I’ve been on several different lines and also know a fair bit about others. Take note of what is written about tips and don’t try wangle out of them- factor it into price. Vacations to go is indeed excellent. September is an excellent time for the Med if you fly.
Luxury lines (Silversea, Regent and Crystal) will have a much older and quiet vibe. Akin to 5/6 star hotel. Very expensive. I’ve not been on these. Wouldn’t want to even if I did have the money.
Cunard and P&O are very British and food is not the best imo. Passengers on Cunard more refined Brits but much older crowd. Would only do P&O for the free night nursery for little kids. Cunard entertainment is more traditional - dances, classical music, opera singing. P&O more modern shows. Embarrassing British anthem sail away on P&O! We called it the Brexit ship. Ships are nice but none of the fun stuff! Staff not too happy/ friendly. Do posh dress up evenings very well if this is your bag
RCL and NCL. American, casual, younger crowd, busier and lots of bells and whistles. Excellent entertainment. Food excellent and lots of variety. Better service and more proactive, wanting to please. Less opportunity for dressing up. Generally have huge ships. Huge ships are more fun! And not necessarily more crowded.
Princess is similar to P&O but American. Hal (not been myself) is also America, bit posher, older crowd and more traditional ship feel (teak wood wraparound decks).
Celebrity is more refined, better food and wine, less bells and whistles because it’s not really for kids but sophisticated fun. Excellent entertainment. My line of choice for adults only but can be pricey. No need to dress up but people are smarter any way.
Italian lines offer great value (MSC Costa). Great food if you don’t expect American style dining. Can be chaotic and disorganised. Quality of entertainment is less but larger ships do offer greater variety. Very good for families. Good for dancers. Stylish and glitzy ships. Casual passengers. My first choice for good value. Lots of kids. American people don’t tend to like food or European style service and guests, so you’ll get bad reviews from them.
Budget lines. With low expectations we’ve had a great time on pullmantur. They have some excellent iteneriares and are all inclusive with alcohol. Less dining options but food was amazing. Old cabins but clean. Naff entertainment but we made our own fun! Full of Spanish passengers. Marella not tried it, but cheapie brits abroad ship.
River cruises. All about the destinations and scenery. Posh but limited ships. Older crowd. Expensive