It's probably wise to check the heads are genuine R90S and that it isn't an R90/6 in drag. Why not check the frame and engine numbers at the same time, too.
As far as performance goes, if you go down the shops or go for a tour, you won't perhaps notice the difference. But if you like rare steaks and Italian tune-ups, and looks are important, the Dellortos are better. Dellortos take longer to warm up than Bings, which perform better on part choke. Dellorto chokes are either on or off. The throttle quadrants are different between the two types, too, so check you get WOT with what is there.
I've seen an R90S for sale with Bings (some time ago) in a dealer; he took ages to sell it. Like years. Could be whoever bought it is now moving it on. Just a guess.
Parts are obtainable for both types.
FYI I've a couple of bikes with Bings and 3 bikes with Dellortos. I think the Dellortos make the bike go better on the pick up (due to pumps) but perhaps do less well on fuel unless you have them in tip-top condition. But who cares, because they aren't used everyday are they? I wouldn't buy an R90S with Bings unless I was going to put Dellortos on it, but that is just me.
