Yep, the first Adventure Island is Wonder Boy with a different name and main character. After that both series went in completely different directions. HG101 has a pretty good write-up about the licensing issues - http://www.hardcoregaming101.net/wonderboy/wonderboy.htm
Adventure Island is essentially Wonder Boy, yes. The other Adventure Island games stick to or expand on the original Wonder Boy platformer template but they're all original Hudson games that have nothing to do with the original developers or the Wonder Boy series.
As for Wonder Boy, from the sequel on (Wonder Boy in Monster Land) it evolved into action-adventure/RPG series that doesn't have a whole lot of resemblance to the original game.
Thanks for the info, guys! Based on the HG101 link, it appears the SMS Wonder Boy VC is the best way to go. Is that your expeirence as well? Any opinions on the quality of these respective series? Finally, when I do get to Wonder Boy in Monster Land, any significant differences in the SMS and Arcade versions (both are available on VC)? Thank you!