Torombolo,
I have used all three of the engines mentioned, I would rank them in this order. O.S. then Supertiger and then Thundertiger. The Supertiger puts out more power than any Thundertiger I have ever owned. The ST is a steal for the price and performs very well. To me the one thing about the St that I do not like is the huge muffler. If you are trying to save money the ST is the way to go, if you want the best then buy the OS line esp. the FS Surpass series. I have never owned Satio but users always are raving about them. They are so expensive however that I have stayed away due to price alone. If you have not purchased already remember that the .90 ST and the .60 are the same size and weight. Same with the OS and other brands. Might as well take the extra hp that is avaliable. Im not loyal to any of these brands just giving some insight from years of using these items. Im sure everyone has their own opinions and they may vary from mine.
Later,
Brad