I own single and double tanks, and am happy diving either. I have a pretty good set of criteria for choosing one or the other . . . If the dive is below about 80 feet for any significant period, it's doubles. If I'm diving 25/25, it's doubles. If I'm diving with a new diver, or an unknown diver, it's doubles, even if the dive is shallow and familiar. If the walk from the car to the dive site is unmanageable in doubles (a couple of our local sites are, for me) I dive a single tank and don't dive with an unknown buddy.
For dives in the shallow range, with known buddies, it's single tanks all the way.
Caves, of course, it's doubles.
How do you sort it out?
To solve the walk-from-the-car problem, for a long walk, I will set up a tarp, and haul my tanks out to it, including a small wooden ladder that I have modified into a bench so I can heave my twin tanks up onto it, and then get into the shoulder straps that way. So the walk to the car does not need to be a critical deciding factor between singles or doubles.
I have not dived single tanks in a long time. However, if the beach is too steep to easily negotiate with twin tanks on, then I would consider a single tank.
It is just too nice having twin 72s on for the beach, and 2 of everything, like two first stages, two second stages, and manifold with an isolator valve, which is the main benefit of a twin tank system. (Twin 130s for boats, which are heavy monsters.)
On those rare occasions when I do dive with a single tank, I virtually always have a 40 cu ft stage with me filled with EAN 50. Thus, again, 2 of everything, just no manifold and isolator this way.
Depth is not really an issue for me. Depth only determines the available NDL time. With twin tanks and 2 of everything, virtually nothing can go wrong during an NDL dive at any depth, which cannot be easily resolved simply by shutting down the isolator valve.
Regarding unknown buddies, twin tanks won't help you much. The mostly likely problem with an unknown buddy is buddy separation, in which case a DPV to help you look would be best. And the worst case is a panicked buddy who takes both of your 2nd stages as well as your mask off. Twin tanks won't help you then either. The main prophylaxis for dealing with an unknown buddy is to make sure you are the stronger.
Once you get used to 2 of everything, you become spoiled, and there is simply no going back anymore. I am thinking about selling my single steel tanks, since I hardly use them anymore. I am guessing the local paintball crowd would be the most interested in them.
Additionally, my fiancee makes sure that I dive with my twin tanks anytime I dive solo. I explained to her how 2 of everything works, and therefore she holds me to it for solo diving. If I want to dive with a single tank together with a 40 cu ft stage, I need to find a buddy. She is not interested in scuba; we do other things together, like hiking, jogging, tennis, etc. But sometimes she sits on the beach in her beach chair under the beach umbrella and reads, while she waits for me to finish my dives. If I did not have twin tanks on, she would be too worried about me diving alone.