Both models have nearly the same chamber depth, diameter, overall weight and design of bulldog shape. One noticeable difference is the style of the bit. Besides that doing cross comparison between the two each is made for different lines of Savinelli model groups.
504 and 510 both come with straight and saddle stems. 510 comes in KS and EX sizes, so it's basically just a larger version of 504.
Same thing Savinelli does with their 313 vs 315. 315 is always KS or EX and just a larger version of 313.