The 510 is quite a bit longer/wider and taller than the 505.
Dont know what size horses you're planning on carrying, but I wouldn't want my 16.1hh IDx in anything with a lower roof than my 510. She's got lots of room for width and length (in 6'9" rugs).
Thanks. Those measurements seem massive somehow.
I’ve already got a 510. I’m thinking about converting it into something else now my horses are retired and we rarely use it. The company that does the conversions usually converts 505s and I was trying to work out how much more room I’d have...