It probably partially depends on how you buy and sell equipment - if you bought the 971 used and can sell if for not too much of a hit and buy a 991 used for not a lot more. I wouldn't think the performance improvement would be all that big, but if you do it wisely and don't mind the steps involved, moving up a bit at a time can make sense.
Personally, I'd sit tight with the 971, a fine player in its own right, and let the next generation of digital hit the used market and/or come significantly down in price. If you could get up to about $1500-2000 used, the difference would probably be much more pronounced - maybe an Arcam FMJ23 or Wadia 830. -Kirk