the twin aisans aren't a bad setup when running right. they will give more grunt in the low to midrange than a 32/36 dgv weber, and have similar top end power.
twin aisans came out on ke15/17 sprinters, and some ke25 SL coupes, also they were on ke35 SR coupes. they were fitted to K-B and 3K-B engines. they give heaps more go compared to a single stock aisan on a modified engine.
SU's are nice all round carbs. probably down on top end compared to twin sidedraft webers by a few percent, but give better all round flexability, drivability and economy, SU's are better suited to a driver.
sidedraft webers are best suited to competition engines, where you are going flatout most of the time. dellortos are almost identical to sidedraft webers but have better designed progression circuits, so offer a little better drivability. with their fixed choke design, these type of carbs work best over a narrower powerband than other carb designs.