I live in a country that's hot and humid all year long . The maximum temperature can go up to 37°C.  so I'm contemplating on which I should use. They ordered a onesie fondant cake for a baby shower. Could you also give me other tips since I'm still a newbie. Thank you so much! :)

Ganache is always better than buttercream, unless you use shortening in your buttercream, If you use ganache make sure you use a high ratio of chocolate to cream almost 4;1 or 5;1 depending in temp. I have also used 6;1 when it was very hot. Keep it in a cool room and work on it. Maybe a room with AC in it? Because at such temps nothing but hi-ratio shortening buttercream will work. 

As for tips it will be good if you are more specific, because just tips can mean anything.