I picked up a 1998 ultra classic last fall and love it. This spring I looked at a 2013 and a 2014 ultra limited. My evo has 68000 miles but runs great. Should I sell it and which ultra should I go with?
The Limiteds have the heated grips and the liquid assisted cooling for the heads. There may be a few other features but nothing huge. They also are offered in completely different paint schemes.
Other than that, they are the same bike. They are going to ride, run and generally perform about the same.
If you enjoy the 98, and it is generally still a reliable bike, you might think about sticking with it.
As was said, the new ones are nice, but the general experience is pretty much the same. The only bike difference I noticed when I went to the 2011 over over 2005 (which I think would be about the same bike as yours except the drive train) was that the 2011 feels longer. It is generally more comfortable for my 6'4" frame. There's less tank hump.
As others have said, the difference between the model years is minimal; refinements actually. You will probably notice the difference in rear tire width on the newer models helps to make the ride feel more stable and you may even notice how the new frame design feels a little tighter versus the older model. A larger engine displacement with a 6-speed transmission are nice upgrades too. New toys are always fun though and a few new farkles can be ever so enticing. Good luck with your decision.