I don't think RF is considered a spin-off of HM anymore. I remember reading somewhere that the devs decided to scrap that idea or something after 1 came out, which is why only the first game has the original Japanese title for Harvest Moon on it, but oh well.
Anyway, Sunshine Islands is great but for some reason, it doesn't feel as fun as Island of Happiness though. I guess it's because IoH has more stuff to do even though SI has you go around collecting sun stones, which feels more like a chore than something fun. I mean, getting most of them are either repetitive (Giving each person a total of 5 gifts to get one sun stone from them) or just plain dragging (Using each Makers 100 times to get sun stones from them). However, SI does have its advantages such as improved gameplay and button controls for those who hated the touch-screen only controls in IoH.