Well.. fords suck just so you know.(slightly kidding. bias to Chevy here) Considering you don't already know the differences, I'm not really sure if I should get all technical and you not understand what I'm saying so.. I'll try to keep it fairly simple =]
The 351C and 351W tend to have a lot more performance and extra parts out on the market compared to the 351M. Seeeing as both the C, M versions stopped production in the 70/80's. There are certain interchangeable parts between the M and C though. I'm saying this simply because, if it's one of the final deciding factors, then take into consideration an older engine is always going to need at least some maintenance/repair so be aware of the options out there.
A 351M is a destoked 400, with a dry intake manifold. Destoke engines tend to be more power engines and produce more RPM's. IMO, 351M's are only really good in an average car and even then I'd pick a 351C over it if the choice was available. They are decent and can be built to make good power, but they have known oiling deficiencies. The higher the mileage gets(70k+), you might end up hearing some bottom end(bearing or connecting rod) knocking going on once the oil gets hot enough. the It can be fixed during a rebuild though.
351C used to be a lot more common in racing because it was the better engine out of all three. It has a better oiling system than both the M/W's which with the 2.750 main bearings and a harder cast-iron crankshaft compared to the Windsor. Also the 351C cylinder heads, were much better suited to performance applications than any production Windsor head. Though with the popularity of the Mustang 5.0, the Windsor production has negated that as they've been improving throughout the years via aftermarket parts. They did stop making 351C's in 1974, just so you're aware. 351C still is a better high rpm racing engine.
351W is a better truck engine as it's been in all Ford Trucks from 83-997. . It pretty much is a tall-deck version of the 221/260/289/302, and slightly wider. The crankshaft has 3.000" main bearing journals, the larger main bearing journals are good for truck engines. If you're looking for a higher performance engine, it will cause oiling deficiencies; even with a high volume pump, it's difficult to get enough oil to cover that large of a surface area.
TL;DR
M=Meh
C=High Performance
W=Truck
Hopefully I didn't get any information wrong, but if you still want more input I can probably ask my dad since he knows a hell of a lot more about various engines than myself. It also would've helped to know what you were looking for besides the differences, and if it was in a truck or car lol.
