The 351W has the best shortblock, the 351C has the best (factory) heads.
Back in the days before aftermarket heads came out, the 351C was the only way to go. The 351W (with heads originally designed for a 221ci engine) had a built-in case of asthma! They were considered 'door stops' and -might- be found in your tow vehicle, but NOT your race car!
Times have changed. Nowadays a person can buy a $500 3.85" stroker crank, use the stock 351W rods, and a set of regular 302 pistons to build a 393ci windsor. Top this with a nice set of aftermarket heads, and you've got a real budget stormer on your hands!
I'm not taking anything away from the 351C. It can STILL be built to make SERIOUS power. The heads are capable of producing more power than the shortblock can handle! It has acquired sort of a 'cult' following, like the FE... and people STILL stand up and take notice when someone says their car is powered by the mighty 351C. It's also a DAMN PRETTY engine when it's all dressed up!
In the end, with all it's modern aftermarket support, it's tought to beat a 351W stroked out to 400+ cubic inches. The selection of heads, intakes and other aftermarket parts for this engine seals the deal.
BUT, if aftermarket heads are out of the budget, or a stockpile of 351C parts exists in your garage, it's a great engine that can make SERIOUS power... with mostly all factory parts. And do it in STYLE!