Having spent many hours instructing in 5.5m Destroyers and Ocean-Pros, and also having owned a 6.3 Ocean-Pro from new, I would always choose the Ocean-Pro.
Stretch to the 6.0m one if you can, and the Suzuki DF 140 suits it well if you have not yet decided on an engine.
I agree with cypman. Are there any particular reasons for wanting under 6m?
I had it in my head I wanted something around 5.5m as that's what I'd always had but when I tried a couple of 6m plus boats there was no going back. Had my OP 6.3 with Suzuki Df140 for 3 years now and, for me, the package is perfect. Excellent sea keeping and a very dry boat.
I thought the main difference was the rails, but I am open to correction on that.
Ironically whenI did my hull I found under the antifouling a huuuuuge repair just ahead of the transom! (Previous to my owning it a sailing club used it as a rescue boat so I guess was beached regularly)