Marilyn,
I'm fond of saying that "I'm in my 8th year of 1st year Mandarin". Cantonese is by no means going away and I encounter more of them than Mandarin at times. It seems that most of the southern Chinese are "into Cantonese". Mandarin is northern, but official, so it is growing in all areas. It's a mixed bag, with Cantonese being practical and Mandarin working, too.
For my money, go with Mandarin; the new generation will have familiairity with it from school. It is also easier to learn and follow. Cantonese has so much slang and "local effects" (about 110 versions of it) that it can't be doumented. It seems to me that the Cantonese don't publish much and let the "Mandarin's" do anything with the outside world in written form.
But don't forget Hokkien/Taiwanese, Shanghaiese & the western language groups and a few others. Good luck. My girlfriend speaks 7 languages and occassionlly can't talk to some of them.
The Chinese are really down to earth people. I expect the JW's will make real inroads there, if they get really going inot a "push". But the Chinese will have a surprise for them; they like to do things their way.
Mustang