Depends on the jobs you're looking to get.
Cantonese sure helps if you're working WITH locals,
Mandarin is a huge ASSET if you're talking with China.
I am told by many that Mandarin is more important,
but those people already know Cantonese and I think they overlook the value and practicality of knowing Cantonese.
Sure Mandarin is a huge asset - if ever you go to China or you know enough Cantonese...but I think if you're in Hong Kong, it sure helps to know Cantonese.
I think it would be a shame if you worked in HK for a couple of years and did not take up Cantonese. I have met a few expats who have reflected on their past few years in Hong Kong and regret not having picked up any Cantonese. It's possible.
Think about it - basic Mandarin alone won't get you the sale from China - but basic Cantonese can help you get places by Taxis and keep your job.
My two cents.
I say learn the local language (Cantonese),
THEN look abroad (like learning Mandarin).
Read more here:
learn Cantonese or mandarin??