That's the frame material, one is chrome-molybdenum steel and the other aluminium.
Debate over which is best has raged for decades without the end in sight. Maybe Dahon makes 2 identical bikes to target both market sectors.
There has been a few reports of the steel one snapping its frame's main tube in front of the hinge. Breakages of this kind is ironically expected more of aluminium.
Steel will be a bit more springy while aluminium is stiff which may or may not translate into a harsher ride for the aluminium-framed bike. You probably won't notice a difference though.
It comes down to which color you lik better, and if you want to drop the extra cash for a bit lighter bike. If you are going to lug it around lots, you'll probably wished you bought the lighter one instead. But it's only 10% so even there you may not notice much of a difference.
Some people say the pea-soup green is fugly, but some of those same people drool over a Surly Long-haul Trucker frame which is the same color.