Eyebrows, once again you are proving your idiocy. You even started out giving good advice and then you just went all blooming lotus again.
To answer the question, Mandarin is the national language. Even in the south most younger people will speak Mandarin and Cantonese. The only real reason to learn Cantonese is if you are studying a southern system when all your terminology will be in Cantonese anyway.
Cantonese travels a lot further than the "deep south", but Mandarin is probably better overall.
As for US Chinatowns, it's like with most Chinatowns in the west - they are populated mostly by Hong Kong and Guangzhou ex-pats, and Hong Kong is primarily a Cantonese speaking area. Although most people there will also know Mandarin.
Of course, a lot of ex-pats are now Taiwanese, so there's another version of Mandarin to consider.
The bottom line is that most Cantonese speakers will have at least some Mandarin. The opposite doesn't apply. So there's your answer really.
"i can barely click the link. but i way why stop drinking .... i got ... moe .. fcke me ..im out of it" - GDA on Traditional vs Modern Wushu
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but what if the man of steel hasta fight another man of steel only that man of steel knows kung fu? - Kristoffer
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How do you think monks/strippers got started before the internet? - Gene Ching
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Find your peace in practice. - Gene Ching