I'm no expert, but experimentation may be in order, rather than just getting the most or least expensive.

I switched my OS FX .46 from a Fox glow plug to an OS #8 and it made all the difference in the world.

With the fox, the transition to full throttle caused sputters and the occasional dying. I also had a dead stick every four - five flights.

with the OS, I haven't changed the plug in six months, the throttle transition is smooth, and it hasn't deadsticked the whole time.

My point is you may want to experiment, not that OS is better tha Fox.