My experience:

SU Pros: Factory equipment tested by Volvo which means they perform well under varying conditions including cold weather. Very adjustable, meaning more potential performance & economy if setup and tuned properly and also because of equal length/shorter manifolds.

SU Cons: SU's require more tweeking than a Weber to keep them running optimal, are more difficult to tune (the DGV weber works out of the box and has one mixture screw to adjust, two SU's mean 2 mixture adjustments, they need throttle synching, 2 float bowls to worry about, hydrolic fluids & levels, etc.), wear/get dirty faster & require more frequent rebuilding than a weber. SU parts are not exactly everywhere, and few know how to rebuild/repair them properly (special tools are needed to work on them and when throttle shafts need to be replaced, it's isn't exactly something you can ask for at your local Auto Zone. Meanwhile, you can buy a brand new weber from many sources, no need to worry about finding a rebuilding specialist). Worn/dirty SU's have a tendancy to leak, potentially causing an engine fire (Webers are less prone to leak).

Weber Pros: Piece of cake to setup and tune. Generally rock-solid, not much tweeking or maint. needed. Easy to rebuild (no special tools). Smoother idle because of single carb/manifold design. The enjoyable feeling/sound of the secondaries kicking in near full throttle.

Weber Cons: The Jury is out on whether a Weber can match a good set of SU's for performance & economy (however even an old dirty Weber will run circles around a improperly tuned set of SU's). That annoying "sucking" noise it makes with that tiny/cheesy K&N air cleaner. My experience has been that the DGV Weber is not well suited to cold climates (my Weber'ed Volvo's always ran poorly in -40 degree weather). Throttle travel is shorter so the gas pedal is adversely effected (shorter travel). Non-factory appearence.

Recommendation? For a daily driver I'd choose the Weber for it's relaibility/less maint/ease of repair. For a show car or Sunday cruiser (and because I like to tinker and own some SU tools) I'd go with the SU's.