Get the OS.

The MVVS might or might not be more powerful in theory, but the OS is far more reliable and will last longer.

It's no accident that the most common engine seen in B and 2610 class combat is the OS .25FX.

If you want more power, look at the Webra .25, it's probibly the most power for the money out there. Some guys have said the needle value is prone to damage, but it's not hard to hack the carb in to a remote needle arragement that is less prone to damage.