Pretty much all the same in terms of typing feel. Differences between Unicomps and early IBMs and late IBMs are pretty subtle, really. Unicomps are slightly lighter in keyfeel by about 5g, but nothing really earth-shattering.
Quality, well, the early IBMs were almost certainly better (thicker steel plate, nicer printing). But again, there's not much in it.
If you want to be safe, get a Unicomp. They're not too far off an early IBM M, and because they're brand new, you know the rivets will all be OK. Plus you can get USB version or a 104-key version with Meta keys if that's what floats your boat.