Hi tk421,
If you are interested to learn Chinese,it is better to start with Mandrain first,rather than Cantonese.This is to get a strong foundation for yourself.If you learn cantonese first,then learn mandrain,its gets much more difficult as you would get confuse about the pronunciation of chinese language.
What is likely to happen is that when they learn cantonese before mandrain,the way they speaks would sound funny.
To give you an idea,I personally have an uncle from grew up in Hong Kong.He understands mandrain,but he really could not speak well probably due to heavy usage of cantonese from young.If he tried to speak mandrain,we would find it difficult to understand what he said.(he could speak English though).
Mandrain is the commonly accepted language among the chinese(still it is not a dialect),and if you speak to an asian or someone with knowledge of chinese,its 90% the person would understand.
