Those of my clubmates who like them claim that a small twinset balances much better on their back than a single tank rig does. A small twinset without an isolation valve is effectively no different from a large, flat single tank, albeit with two 1st stages.
I prefer singles because it makes it possible to swap tanks between dives. Those twins hold only 50% more gas than one of my singles, so if you don't have a compressor available, you're basically limited to one and a half decent dive during a day outing. Besides, a broken down singles rig with two tanks is easier to stow under the benches in my boat.
Tech doubles are, as I've said, usually D12x232 around here, and although they provide enough gas for two good no-stop dives, they're no fun to schlep from the car to the boat, or to carry down to the water on shore dives. And since larger tanks means more buoyancy, the total weight of your rig and your weight belt increases with increasing tank volume