Horse Trailer

Whats the difference between the ifor williams horse trailer?
hi
i am looking in at buying a new horse trailer and was just wondering whats the difference between the Ifor Williams HB505 and the Ifor Williams HB510?
thankyou
From a quick google the 510 is for two 17.2 and the 505 is two 16.2
They are good trailers to tow, very well balanced.
Can you believe this is a horse trailer?