Hi Mick, Welcome to SAU.
I general, cars of all types suffer a drop in sales over mid December till February due to preoccupation with Christmas, holidays, getting kids ready for school and yards clearing out old stock. Over this time, bargain hunters can feast.
If you're going to buy a R33 or R34 I'd buy it through "For Sale" > "Private Whole Cars only" here on SAU. Issues like regular servicing, maintenance, modifications, dyno reports and blemishes are all sorted.
R33 or R34? Newer is probably better and since less R34s were made than R33s, whilst Nissan back in the late 90s were in financial trouble until Carlos Ghosn was given the reins, the value might remain more stable - or it might be easier to resell later on. However in saying that, R33s can't drop much more anyway, so it's a personal choice. Just get the best example you can find - without necessarily trusting the odometer.