I have a Moki 1.8 on my 13-lb Cap. It swings APC 18x6@9400 rpm peak, which pulls 19lb static thrust measured. It's just enough. The same plane w/ OS 160 would weigh 12.5 lb, and would pull about 18lb static thrust. Pulling power may not be as good, but plane will fly better due to lighter weight. Another question to ask is, will the OS balance the plane w/o nose weight? If so, go w/ OS. Just swing APC 18x6W prop on high nitro fuel (15%~20%) to get extra power and transition.