Funny, because I could have sworn that you said to only go by feats, and not statements.
Since everyone is already dead seat on their opinion of this fight(thus explaining this thread's inactivity), I'll explain my reasoning here:The spirit bomb used against kid Buu contained the power of Gohan, trunks, goten, piccolo, and every single person on earth. Yet we see Kid Buu successfully pushing that power back at Goku.
Now I've seen counters to this argument before, and these counters are fallacious. So let me go ahead and disprove the counter argument.The counter to this is that the spirit bomb depends on a good push, not raw power. The reason it didn't work against kid buu, is because Goku was worn out, and didn't get a good push.
My response: Goku was worn out during his fight with frieza as well, in which he used the spirit bomb. Frieza was at full strength, and Goku's energy was depleted. But guess what happened? Frieza couldn't push the bomb back. The only answer to this is that it was the raw energy that got him, not a good push. So this would have to mean the same for kid buu.
As for SSJ3 Goku, he was fighting evenly with Buu up until a certain point. Though I still see kid buu as being stronger, SSJ3 Goku was almost on his level. So the SSJ3 boost is far more powerful than the Mystic boost.
As for planet destruction, take a look at this page...
http://www.nfsupreme.com/hostees/dranet/mangas/showimg.php?file=/Dragon%20Ball%20%26%20DBZ%20Manga/volume27/Db27ch07/db27_104.gif
Goku said that Frieza missed. What did he miss exactly? The planets core.
On the next page, frieza says that the blast made its way to the center. Keep in mind that he said it wasn't strong enough. And just how would it be this difficult to destroy a planet, because didn't frieza destroy planets in his base form?