Your buddy is correct that longer props give more thrust -- at the same power load. However, your TT Pro 40 will turn that 12-4 @~ 13,000 --- 3,000 rpm short of its power peak, & the 11-4 will spin @ ~15,000 rpm -- only 1000 short of the power peak. I think that you will find the 11-4 a better choice. Total thrust will be about equal for the two props, but the 11-4 will give you ~ 20% more hp, 20% more torque & 15% greater prop slipstream discharge velocity (= quicker engine response, more level speed & greater entry speed into vertical maneuvers).