The descriptions I've seen on those tour sites had mainly mentioned the reef sharks but you could encounter either there. So yes it seems that if attacked the Blacktip Shark is potentially more dangerous, but it seems that neither is particularly aggressive. Here's a description of the two from Wikipedia (there are links to sources in each):
"
Blacktip sharks showing curiosity towards divers has been reported, but they remain at a safe distance. Under most circumstances, these timid sharks are not regarded as highly dangerous to humans. However, they may become aggressive in the presence of food, and their size and speed invite respect. As of 2008, the International Shark Attack File lists 28 unprovoked attacks (one fatal) and 13 provoked attacks by this species. Blacktip sharks are responsible annually for 16% of the shark attacks around Florida. Most attacks by this species result in only minor wounds."
"Under most circumstances, the
blacktip reef shark has a timid demeanor and is easily frightened away by swimmers. However, its inshore habitat preferences bring it into frequent contact with humans, and thus it is regarded as potentially dangerous. As of early 2009, 11 unprovoked attacks and 21 attacks total (none fatal) were listed on the International Shark Attack File that are attributable to the blacktip reef shark. Most attacks involve sharks biting the legs or feet of waders, apparently mistaking them for their natural prey, and do not result in serious injury."
