[chryuop]

It is actually neither one. The sound is a middle way, so according to what romaji style you use, you can use either hu or fu (that's what I have learnt). It is easier to teach the fu sound coz some people have problem in undertanding the "choked" sound which stands between hu and fu.
But I admit I am not good with romaji coz I never use it and never used it in the past, since day 1 of my Japanese learning I got rid of romaji. I have bought a grammar book recently, which I think it is very good, that uses only romaji. I am having trouble understanding even the easiest phrase coz I am not used to romaji at all hee hee.