Hello,
if you put two cars next to each other, one with fi and the other with webers, it all depends on one thing :
do you have a good mechanic that knows webers ?
Once a fuel injected car is properly set, you should have no problem anymore, there is nothing to tune or change once it's fine.
A Weber setup is very difficult to set right I've heard, but once it is perfectly aligned it should stay that way, don't start fiddling with it.
From what I've heard the weber cars had more hp than the fi ones, also the sound of the engine is different between them, the carb'ed engine produces some kind of howling noise from the air that's sucked into them.
Note this is all from hear say, I would go for a carb car anytime, mainly because this is the way the engine was originally built, but that's my personal opinion, I know people with a fuel injected Countach that are extremely happy with it.
Kind regards