Wow talk about opening a nice can o' worms.
I think that you'll find this forum is about the least weight conscious of all the
road related forums here at BF.
That said, yes there is a difference. A lighter butted tubed bike will ride better than
the equivalant "gas pipe" model. However that doesn't mean that a carbolite or U08 frame
doesn't ride well. There is something about a steel frame that is magic, maybe the way it
soaks up road buzz, or handles bumps or turns, but most here prefer that ride to the newer
lighter frame materials (well I do, can't really speak for everyone else).
Could I tell the difference with names/tube decals covered up? yah I think so, but not on a
round the parking lot test ride, need to get some miles in on the bike.
Snobbery? Look at the bikes we talk about here Raleigh 3speeds, Peugot U08s as well as PX's,
almost all are old steel frames.
Lightweight is probably less than 25lbs, my bikes average about 20 give or take a few grams.
Lghtweight for the road crowd is sub 18. I don't think anyone in this particular forum is concerned
with carbon fibre bits and trying to shed .05grams off a bikes weight.
Do the new bikes handle and stop better? no I don't think so. Well maybe the braking, especially
if the "vintage" bike has something like campy delta's which are speed modulators at best.

and that's just my opinion, I'm sure many other members have differing ones.

marty