Greg -
1919's are great guns - especially for sustained full auto. They are about the sturdiest thing out there and easy to maintain and keep running. I've seen guys turn their barrels red hot on 1919's and the guns just keep chugging along. M60's are also nice guns but IMHO they've gotten far too expensive - especialy for the E3's. A Maremont or SACO Defense gun is going to run your buddy about $18-24k depending on condition, model, etc. Even a cheap Rock Island M60 will run a minimum of about $14k. Add to that the fact that M60's were designed to be 'throw away' guns after about 100krds, I could never justify the expense; I'd rather have two 1919's for the same price. Sure, you can keep 60's running long after 100krds, but maintenance and repairs are not cheap...nothing about a 60 is cheap.
I put the HK21's in the same class with the 60's. Nice guns, but parts & barrels are just too hard to find and too expensive when you do find what you need. Add to that they are also not designed with a long service life in mind. According to my HK LE catalog, even the 21E (with the receiver reinforcing rails) only has a minimum service life for the reciever of 60krds. Well heck, if 60k is the minimum, how much can the max be - 80krds, 100krds, maybe 120krds at the very max? I guess its not a problem if you have a sear gun and don't mind having to get it completely rebuilt, but on a receiver gun it could be a huge problem. 1919's on the other hand are more in a class with the FN MAG - they're damn near indestructible.