In just winding down from a "first ever" Cross Country season I am pondering the title question.
I've ran the Boston Marathon off and on about ten times with PR of 2:38. All of these races were preceded by a fall marathon. Now, as a masters age runner the intention is to train smarter and run faster.
A huge advantage I see in XC is the quicker recovery. Last year it took four weeks to build mileage up, which only left four weeks of base training. Then it was onto a 16-week marathon conditioning program. It seemed the body wasn't prepared for the work.
With XC, the miles can stay up at base level with every two-three weeks a pullback as a mini-taper for a race. The races serve as a built in rest mechanism and a quality workout.
Would a Fall Marathon or Fall Cross Country be better to produce a faster Boston Marathon?