I own M14 type rifles as well as a BM 59. They are both outstanding rifles. The Tri-compensator on the BM 59 is fantastic. It really helps tame recoil and muzzle rise, not only on full auto but for rapid semi auto fire as well (my gun is semi auto only unfortunately). The BM 59 is shorter and handier than a standard length M14, but then again you could opt for an M14 in Bush or SOCOM configuration.
Compared to the M14, reloads are a little faster on the BM 59 because inserting magazines is more of a straight in motion. The downside to the BM 59 is of course availability, and cost of magazines which run 75 bucks each. The other downside is mounting optics. I have never seen a BM 59 with a scope mounted on it. I've read claims that Beretta did make one, but I am not certain about this.
Reese Surplus is now out of BM 59s. There are BM 59s out there, but you have to hunt around and often walk away because prices are sometimes outrageous. Unless the rifle comes with a ton of accessories and extra magazines anything over about $1800 for NIB or like new is a rip off in my opinion. Receivers can be found to be marked either Beretta or Springfield on the heel. Springfield bought a ton of parts Beretta had lying around. Some receivers were already stamped with Beretta markings, and some were not stamped at all. Springfield did some final machining in the USA on some of the receivers they imported. The receiver on my rifle is marked Beretta.
Just watch out for the BM 59 clones and fakes out there. There are lots of them chopped together out of U.S. M1 rifles. The quality varies wildly on these conversions, with most being garbage. Hope this helps.