The reason why apples are sweeter is that they have more fructose. Fructose has a higher sweetness rating than that of other sugars. The GI will depend on how ripe the fruit is also. Sweetness does not correlate to a higher GI rating and/or insulin spike. Fruits are healthy for everybody to eat, just don't build your entire diet around them. A few pieces of fruit are not gonna make you fat, that will depend primarily on your total calories consumed each day. Also keep in mind that fructose will glycogenate the liver and not the muscles. Both bananas and apples have their benefits and drawbacks, you just have to find which one fits your needs better.
BMJ