When I was looking I found the common specs were 60"x20" with 500lbs capacity, fold up or not. So I went with a folding one on amazon $118 CAD free delivery. A little assembly, but very solid and functions well so far.
It's great having the extra storage capacity, especially for things I don't really want inside the rv. Like my 14 gal gas caddie for those long lonely northern roads. I f I were to take another spare I would put it on the cargo rack, right now mostly firewood and outdoor camp stuff.
I also like the fold up feature, I park deep in my driveway right in front of garage doors. After unpacking the rack I put it in the vertical position and can walk around the rv with out stepping over the rack everytime. And days like today where I took the rv in for tires, it's nice to have it up and not extending the rv too much.