Dutch And Flemish are the same language, the difference between both is similar to the difference between Spanish in Spain and in Argentina, or English in England and US. We have different expressions, vocabulary and a (very) different accent. Dutch movies in Belgium have subtitles and vice versa, although the news in both countries we would understand without any problem, both Dutch and Flemish.
Native English speakers will find Dutch a lot easier then for example Spanish speakers. Dutch on the other hand is closer to English (in pronounciation) then Flemish is (which is closer to French in pronounciation). The Dutch will always speak English with a better accent then the Flemish, the Flemish will speak French and Spanish with a better accent then the Dutch....
If you think about moving to Belgium, it would be more convenient (and easier) to learn Flemish, but if there is no other option, learning Dutch would be better then learning nothing at all...