welcome to Cruise-Chat.com.
As for your question, it's like asking whether apples or oranges are the better fruit. The "Galaxy" would usually be considered the more upscale product compared with the "Costa Victoria", but that doesn't mean it's the "better" product for you
Costa has a more international passenger mix with many European passengers (Italian, German, Swiss, French, Spanish etc.), while Celebrity has mainly English-speaking passengers. Costa offers a more relaxed experience, while Celebrity is somewhat more traditional.