The older 952/3 mechs still work very well by modern standards ~ very lightly sprung so have a nice light action at the lever. This is also a disadvantage at times though.......it takes less in the way of friction and cable contamination to make the shift deteriorate.
The newer 972 mechs have stronger springs, snappier action and are less prone to going off song due to dirt ingress. Also, the Shadow design smooths the cable run, by shortening the lass piece of cable outer, which is a good thing. Shadow has a lower profile, making it less prone to rock strikes too.
Overall, if you want a mech to work and keep working through crap with low maintenance, go 972. If you're more concerned about "period correct", the go 952/3.