racer sums it up nicely....
IMO the ski's if both are pristine, are worth "about" the same, with the gsx getting a slight edge in price +300 because it suits a larger audience, and another 300 because it has a new engine. so i think the spread between those two is about 600, then add $100 to that right off the bat for new mats.
i'm sticking with my original advice, try to sell your ski for best value, as close to the end of the season as you can, and buy a replacement ski over the winter and sexy it up with your free time.
the only thing i'll disagree on is price, but i'm florida biased, i don't thing either one is worth close to $2500. my buddy just sold his 96 xp for $1600 and it was the prettiest dam ski you'd ever see with 30 hours on a complete rebuild, factory pipe, xo prop, proks, custom paint and much more. now granted if he had time to sell it properly (was in a hurry) he could have gotten 2K , but not a penny more, they're just not worth that much.
in general, in my area, a 96 xp running spot on is 12-1500 all day long, and a 96 gsx is 15-1700. based on what little I know about your's and the xp,,, i'd value the xp at 1200 max, (needs work and unknown bottom end condition) and the gsx at 1900 due to new engine. you can add value to either due to location but IMO the spread is the same,,,
for me to pull that trade, i'd accept nothing less than $500, and i'd probably hold out for 7.
sell your ski for 2100ish, and buy a decent xp for 900-1200 and fix her up over the winter. that gives you 7-900 of wiggle room. if you find a junk xp (needs engine) then pay about $450-600 and then you have even more wiggle room. i don't care if a hull is dirty, they generally clean up real nice,,, but I don't like unknown cranks.