Märzen is the older, amber style formerly associated with Oktoberfest. It is very well attenuated, made from some light Munich or Vienna malt as well as Pilsner, crisp, dry, drinkable. Since at least the 1970s the beer served at Oktoberfest has been what is called Festbier to distinguish it. It is basically an export strength (5.5% ABV, about like the Märzen) Helles, so it is even MORE crisp, dry and drinkable. The point of any Oktoberfest beer has always been to be eminently drinkable. You spend all day and night in the tents pounding away until the brewery has all your money. The American style that has emerged in the craft era is characterized by a lot more strength in many cases, a lot of caramel/crystal malt and darker Munich type malts, and has a very full, sweet impression. Sam Adams is but one example. I haven't found a single American made beer this year that is in either German style, and it's more confusing that they interchangeably label them Festbier, Märzen, Oktoberfest, or some variant, without seemingly having any clear meaning. As for this year's Sierra Nevada entry, it is baffling to me. Not only not at all German, it isn't even in the general American style. But I suppose very SN. Gobs of crystal malt, hardly very drinkable, but with a hop character that just doesn't balance at all. I am shocked that Bitburger allowed their name to be attached to it. Their only contribution was some of their contract grown hops, no input on formulation.
One key to understanding German festival beers is that they are meant for festivals. For drinking in quantity, and to wash down lots of roast chicken and pretzels. Over the centuries, they've gone from brown to amber to pale, but essentially they have always been just a slightly stronger but even more crushable version of whatever the mainstream style of the day was. I'm not well versed in the history of the American craft style, but it seems to me the inspiration was more along the lines of a rich, warming seasonal to sip on chilly nights in the pumpkin patch. An entirely unrelated class of beers, but legitimate in their own right.