I would think the metric century is easier, especially for newer riders. I'm training up for a full century now, and even though I do 18 miles a day, I pushed pretty hard on my 50 mile ride. I can say that getting back in the saddle for another 50 miles the next day would have been REALLY hard.
So, IMHO, I'd do the metric first, then the back to back second.
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Cycle tracks will abound in Utopia.
~H.G. Wells