All the strange, oddball sounds in the world will not make up for a missing or bad basic set of sounds. There are a few exceptions to this rule, namely techno and dance, which have a much greater emphasis on samplers. Built-in speakers on a keyboard are a waste of money for a performing musician. Built-in speakers will never be loud enough to play over a drummer, will sound bad, and cost money for the manufacturer to add to the unit. A serviceable keyboard amp is a much better route.
Many keyboard amps have a vocal mic input that sounds fairly good.
However, if your band has a PA system, try plugging your keyboard into the PA board. Your keyboards will sound good and you can spend the extra money on a better keyboard instead of an amp. Plugging your keyboard into the PA also has a side benefit in that it will allow you to play stereo sound patches that sound better than mono patches. Now lets talk about the PA or vocal system. This is normally the hardest part of band equipment to afford, but one of the most important parts of the sound.