Both of them are OK, one of my colleauges' first name is Cai(柴) as well , Wei should be spelled to "卫 or 韦" instead of "喂",In my opinion,Cai wei si is better than Wei Wen Chang.

I have to say both of first name in your name option are not the chacactor often seen as first name. And WEI喂 means feeds in English, and in some case people will make the sound WEI when he need someone's attention. And I suggest you take the former one, Chai Wei Si.