The difference between the Legacy and the S-500 is the pickups - that's pretty much it. The Legacy has Alnico V pickups and the S-500 has the G&L MFD pups which are ceramic.
G&L's are excellent guitars, especially for the relatively low price. The PTB tone controls are AWESOME (they're Passive Treble and Bass instead of just regular tone), the mini toggle allows for the combinations of B+N or all three pickups at the same time, the options available at really low prices are a huge bonus, too (locking tuners for $35 for example). And IMO the general quality of the guitars far exceeds that of Leo's more popular company.