You may run into poly issues within a year and you may not. That really depends on how fast you progress with your learning, and what all you venture into with the S-500. If you know you're going to be at it for a several years, and if money isn't the issue the extra cost IMO would be worth it in the long run.

The difference of 64 note poly and 96 note poly is 32 notes. Once you start getting into creating your own songs and heavily into the arranger play you'll start to understand where poly can by an issue. The S-500 is a fine board to start out with. It has great sounds, but lacks some features you may want in the future. So investing in the better borad now may be more beneficial to you down the road.