Two R15 Batts in a 7.25” Space vs Compressing an 8.25” inch R30 Cathedral Batt? – GreenBuildingAdvisor

What are your thoughts about using two R15 batts totaling 7 inches depth and R30 in a 7.25 inch cavity vs an R30Cathedral batt which is 8.25 inches that gets compressed an inch down to approximately R27?  

The two R15 batts would come out to about $.40 cents cheaper per sq ft and has a better overall Rvalue (R30) than the compressed R30 Cathedral (R27).

This is DIY so don’t factor any labor difference. There will be no exterior insulation. Blown insulation options are not being considered. I am not going to spring for the extra cost of R30 mineral wool.  

I believe the two R15 batts is the overall best option, but is the possible .25 inch gap on the warm side of the house big enough of a deal to go with the R30 Cathedral due to potential convective looping? 

Thanks!