Every material for drivers; paper, aluminum, kevlar, carbon fiber, nomex, ceramic, magnesium, beryllium, etc.... all have pros and cons. And incidentally, more people than SEAS and Scanspeak make great drivers.
Each material has different characteristics that behave differently in different applications. Paper is not inherently good or bad. Paper cones are not simple card sock cut into a cone. The formula is typically a carefully guarded secret and they can deliver elite level performance. I find the paper drivers I use (Eton's Orchestra) offer a tiny bit of coloration which makes them engaging and incredibly musical. In the right application, paper drivers can simply be extraordinary.
In terms of accuracy, I agree that magnesium offers some distinct benefits. I prefer Eton's magnesium driver to SEAS more because it is much easier to work with. The SEAS is very difficult to get the crossover just right while Eton's Arcosia works with simpler crossovers and delivers and incredibly smooth yet accurate sound. I actually prefer Eton's paper drivers compared to Scanspeak as well. But, this is based on my designs and my experience using lightweight but heavily damped cabinet material. The EXACT opposite of say Joseph Audio or Wilson who are producing high mass cabinets.
In the end, a speaker is a complete system and you can't take a driver and say it is "the best" without the context of cabinet and crossover. A great driver in a bad cabinet or with a bad crossover will deliver a bad speaker. A modestly priced driver integrated with the right cabinet and the right crossover can be insanely good and in the end, it is the complete design that matters. Individual parts should be taken in the context of that system.
