Training & Nutrition - metric century vs. split century

I've been moving through my list of goals for the season, and am wondering about the order of a couple of mine. In your opinion, which is easier: a metric century or a split century (back-to-back 50 mile days)? In what order should I accomplish these?

A metric is only a half-century plus 12 or so miles; practically nothing. I would do a metric one day and 50 miles the next and kill two birds with one stone. Or two stones. Or, something like that.

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.

I have to agree that doing back to back long days is harder to do than one extra long day. I've done 45/35mi weekends that really pushed my endurance the second day. I've also done 64mi rides where I was completely wiped out the next day, all day. I'd probably go for the single long ride, then focus on shortening your bodies recovery time to do the double 50's.