I got the 5.1, nabbed a 6 off of eBay, and then decided I needed a 5...
They're all great users and it really comes down to a personal preference for balance point and blade length. I liked that the 5.1 and the 6 are the same OAL but the 6 had more blade length which made me think the 5 would be slightly lighter, just as robust, and have all the cutting ability of the 5.1.
I got my 5 today and it's lighter, just as robust, and should have all the cutting ability of the 5.1 but I haven't tested that just yet. SOON though!
B