Can I vote for none of the above?
How about a high pressure steel 80? They're are full at 3500psi, and are really small. Small doubles are going to weigh a bit more I would think.
The profile of skinny twins may fit slightly closer to your back, but the combined profile drag is actually higher by virtue of more surface area of two tanks. Plus you have the issue of now VIP'ing 2 tanks instead of one, and two hydro's each 5 years.
Not to mention the added expense....BUT, I agree, there is a certain cool factor of running small twins.....U.S. Divers had a slick aluminum triple set we see on E-bay now and then that you could pick up for a couple hundred $.
The US Divers triple al 30's did not use a DIN valve though.