We'll assume your talking about comparing a Weber DCOE and Dellorto 48 DHLA.
Power band.
The Mikuni will have less top end due to its size than the Dell or DCOE.
Otherwise it should repond about the same thru the rest of the power band.
If you use a wraparound intake (RB or Lake Cities) the torque curve will
be low enough to make the carb very streetable. If you use the short
side intake from Atkins it will move the torque up about 1000 rpms.
Gas Mileage.
Really? You care
You can expect around 17/22 for a stockport with a
good tune on the carb.
Durability.
As long as you use good fule filters and a correctly size air filter (allowing
at least 1.5 inches above the top of the air horns) these carbs will all last
a long time. Longer than the engine or car I'm sure.
Ease of tuning.
The Dell is the easiest and have the most adjustments. You can change the
jets while the car is runnning
The Weber and the Mikuni are almost as
easy but require you to shutdown to manipulate the jets.
With all the carbs the big thing to address is setting up the fuel delivery to
be at the correct pressure and flow. They all operate at fairly low pressures
but require a good 40 gph pump. The Dell likes pressures at 2.5 psi, the
Webers like 3.5 to 4 and I think the Mikuni are the same. This means you will
probably need to get a good fuel regulator (return type recommended).