For a given engine, a .15 will produce more power than a .12. But there are different engines out there. The .15fe would produce more power than the .12fe, but that is where the comparisons end. A Picco or Novarossi .12 will put out about 1hp compared to about .4hp of the fe. But a .15 Picco or Novarossi will be more powerful than the .12.

So basically, get just about any other .12 engine on the market and it will blow the doors off the .15 fe.