My Etherwave is a Big Briar, too. I've been fortunate enough to try out the Etherwaves of others, of varying ages, including ones right off the assembly line at Moog in November in Kansas City (so they were the absolute newest ones up to that point). Truthfully, the only real difference I can discern is all timbre related. The wave form and brightness knobs create slightly diffrent textures than my older one. Actual "playability" is relative, because you CAN tweak your old Etherwave fairly easily. I've gone in and re-tuned the pitch and volume oscillators with the result that the machine has a wider range than the current Etherwaves, and a sharper response (when needed) at the "threshhold" right between zero beat and the first sound.
The new ones (there were five of them set up) were all factory settings and surprisingly consistent -- the threshhold on all of them was at about 2.5 inches above the volume antenna, and most folks never mess with that. I've set the distance to be greater, at about 3.5 to 4 in order to be able to get certain effects.
Based on what two or three of my friends say (all of whom have Etherwaves from one to years years old) it appears that they're very stable, less prone to oscillator slippage, and a little less sensitive to environmental conditions.
It's doubtful that you're at any sort of disadvantage with an older Etherwave.