My anecdotal opinion when comparing two similar items: a Keepall from 1993 (made in USA), vs a Speedy B from 2017 (made in France.) Maybe the only real noticable thing is that the vachetta is a little more quality on the old one. It's hard to say for sure since I don't know how the Speedy B will hold up in the next 30 years (not to mention my personal preference is for the very aged patina look.) The hardware and design quality seems the same to me overall (except the B strap, which has different hardware for just the strap). The canvas is definitely not the same, the old Keepall is a little more smooth and buttery and the monogram is a slightly different texture. I wouldn't say that the old canvas is better than the new at all, if anything I expect both versions to fall apart at some point in this lifetime. Obviously that the Keepall is still alive is a testament to its quality, but I don't have any signs so far that the Speedy B isn't made to last just as long.
If it were me shopping for a Speedy B, quality differences between vintage and new would not have any impact whatsoever. If there is even a discrepancy it is negligible especially when you factor in wear and tear.