The Allison is easier to maintain by a reasonable factor. With a Merlin, you have to torque the cylinder liners every 25 - 30 hours. They are NEVER a maintenance item on the Allison unless a cylinder has to be changed. Then it gets torqued to 2,200 foot pounds. The Merlins are reasonable to run for a higly-stressed V-12 and I like the engine, but it is WAY more complex than an Allsion 1710. Neither partitularly leak and history has spoken ... the Merlin is a great engine. Unfortunately they tend to ignore the Allison which, to me, is just wrong.
The late-war Allisons were great performers, even on European fuels which we didn't have in the USA unless we imported it. Normal maintenance consists of carburetor overhauls, oil changes, filter and screen cleaning at intervals and proper starting and operation .... so does the Merlin.
Today, an overhaul for a Merlin can get to $250,000 US ... if they have the parts and they are becomming scarce ... not gone, but prices are incrreasing. An Allison can be overhauled for about half the cost and will deliver great performance for YEARS of reliable operation. I have one friend with a P-51D who got 40 hours out his freshly-overhauled Merlin ... until he found metal in the oil. Had a broken ring. He's back flying again, but the expertise is dying out.
Likewise for the Allison.
Right now, the best Allison overhaul you can get is from Joe Yancey in Rialto, California. Expect to get in line and wait a year, and you get an overhauled, run-in engine that has seated rings, is ready for flight, and will last a long time if operated per the Allison book.
Forget the book and go by "seat of the pants," and NO WWII V-12 will give good service.
Joe will gladly show you how to hook it up and run it, and YOU get to run the engine as long as you want. He can also supply props if required, and has some exhausts if you don't plus available accessories like starter, generator, etc. He has a variety of carburetors that are ready for overhaul.