Break it in with the 12x6. The 13x6 should be good for general flying when the engine is fully broken in. The 12x6 will give a lower maximum fireing pressure and hence a lower load on the aft bearing and big end bearing.

BTW
If this is the new type ASP with the crankcase vent lead to the inlet pipe, I recomment you to vent it direct to atmosphere during breaking in.