whats the horsepower rating and rpms
The HPI .15FE is only rated at .85 HP, the HPI .12R is rated at 1+ HP.
The Fantom .15 is rated at 1.35 HP @ 39,500 RPM.
It is really not realistic to compare figures between two different manufacturers as different methods may have been used to obtain their figures.
In my opinion and from what I've read online, the Fantom is more powerful. The Fantom is also ABC construction, while the HPI .15FE is ABN. I personally have a Fantom .15 on order at the Hobby shop.
In general, for the same brand of engine, a .15 will have slightly more HP and noticably more torque than a similar .12 size engine.
Hope I helped a little.