I'm torn on this one too . *The finish is a toss up, if I am not mistaken the one you're talking about has an oil finish (which I love.) *The headstock on the Prestige is much cooler though (MOP inlayed) and the marker-less fingeboard is very cool.
Here's a few more differences, the S520 has Ibanez "V" pickups whereas the S1520 has QMs. *The QMs, IMHO, are not as good sounding as the Ibanez "V" pickups. *When I had the S1520, I swapped the pickups out (so now I'm way over a grand on the Prestige.)
The neck on the Prestige is incredible, the finish on the back is perfect, very smooth and fast. *No heavy lacquers (or whatever they use these days) on the back of the neck.
Either way, basically they're both great guitars. *If you plan on replacing the stock pickups, and have over a grand, buy the S1520. *But, if you only have enough to buy the S1520, and want to replace the pickups, go for the 520.