DTB1981:
I am looking to get some longer bottom time...
Except they are all yoke and at least one will need to be converted to DIN...
So what the would be the big differences between the two?
As the others have noted, longer bottom time...to do what? If you are essentially a macro photographer, and you want to be able to spend an hour working on photos at 35' or so on a warm reef, thats one thing. If you're going to spend that same hour on a wreck in 90', thats another.
In the case of the former, the single tank is much more convenient. In the case of the latter, I'd opt for the doubles.
Doubles may be set up with a manifold that will accept yoke regulators, using small inserts that screw into the DIN port. You need not (necessarily) convert your regulators to DIN
In terms of big differences between the two, I can think of several. Double 80s balance nicely. They are not severely negative. They provide (with appropriate training) redundancy and enough gas (as Jonny noted) to do either two NDL dives to around 70'-80' or so, or one longer dive with rock bottom backed out. OTOH, annually you must break apart your set of doubles and VIP two tanks, O2 clean two tanks if you're partial pressure blending your nitrox, and then reassemble the doubles - which isn't difficult, but its time consuming and more expensive than VIPing/O2 cleaning one steel tank. Ergo, the PITA-factor is a difference worth considering.
If you do carry a "pony bottle", or bail-out bottle, it is designed NOT to be used (or calculated into your gas plan), but to be held in reserve in case some catastrophic failure were to occur on your primary gas source. If you breath it first, simply to extend your bottom time, you're using it as a stage. This is one thing in doubles, where the doubles themselves provide some redundancy, but another in a single tank, where the practice leaves you without redundancy. If you're using a single tank, it would be unwise to breath off your bail-out bottle first, then switch to your backgas. The double 80s would be a much better option than that scenario.
FWIW. YMMV.
Doc