The 500 and 700 share the same electronics and neck, the difference is the body. 500 is all mahogony (and just waxed, which is why I never bought one as I really dislike that finish), the 700 has a maple/mahogony body with a maple top and looks great.
I own a 300 but have tried the 500 and 700 on numerous occasions. I'd buy a 5 or 7 second hand but not new. The 5 and 7 are finished a little better but not enough for my liking to warrant me paying over twice the ticket price of a 300. Also regards the pups etc, the barts in the 5 and 7 sound fine, in some instances better than the stock Ibanez pups in the 300, however it's still not enough for me to buy a new one.