If you want to indulge in a regular rev fest go for the 748.
It's not only a power thing, a 998 with have a lot more torque to catapult you down the road without having a million revs on the tacho. It may well be a as lot less frantic to ride.
A 748r will get you a fair few more goodies chassis wise, maybe ohlins front and rear and maybe 4 pad calipers.
One factor may well be the cost of insurance, I know of a fair few that have gone for a 748 cos it saved them a packet on insurance, might be worth checking that out to see if that applies to your situation.
Difficult call.
You can check out the specs of older bikes on
www.ducati.com Ray
[Edited on 27-9-2005 by Ray]