Answer: "It depends on the design details more than the behaviour of the engine".
If the engines are equally efficient, then whether it produces more torque at lower rpms or less torque at higher rpms is irrelevant. If they both produce the same power then the fuel burned and emmisions prodiuced will be similar. Combustion efficiency is more important. In low load/low rpm conditions 2V engines can be (but aren't necessarily) more efficient. This is why (together with cost) Honda is still designing new 2V engines and why port deactivation is so popular. In high rpm/high load conditions 3-5V heads help (but are not sufficient) to burn more fuel cleanly to produce more power. Aside from the ease in incorporating multi-valve heads, cam placement and number is irrelevant. Any engine can be designed to be very fuel-efficient at lower rpms, by simply using less fuel and producing less torque. As engine speed increases, power will also increase, until torque falls off faster than the engine speed increases, but fuel-burn and emmisions will also increase with the in rpm. If low-rpm power is adequate for the application, then you can offer a combination of low rpm fuel-efficiency and high-rpm power, when the driver desires. A VVT system can change between a low rpm torque regime and a high-rpm torque regime, but fuel-efficiency isn't the only reason such systems are used. They can be optimised purely for maximum power, minimal fuel use, a balance of both, or alternate between the two extremes. Theoretically timing and lift can be altered independently of rpm, offering either efficiency or maximum torque at any given engine speed depending on the conditions.