I have a couple of HK91s and a V51P. A few things to consider...
The PTR (and it's no longer JLD unless you're buying an older one) 91 is a nice clone with minimal reported problems.
The V51 is a nice clone with a few problems reported lately, but Vector is great at taking care of them and then you have a bad-ass little weapon.
The V51 takes a special recoil rod that is only available from Vector. It has an additional 2" retraction segment to allow for that much extra compression, because the bolt needs to travel that additional distance for cocking/charging the weapon. You can buy a stock from Vector or they will upgrade your stock (any one that you own already or want to get from a 3rd party) for $60 (talk to Chuck). The converted stock will reportedly still work on a 91 or clone.
Many people don't consider the V51 as a real weapon due to the reduced velocity which results from the shorter barrel, but testing on concrete block shows that it is still pretty damn powerful.
The V51 will require registration as an SBR if you want to put that stock on it, unless you have a sear pack.
The PTR 91 will be very front heavy with a retractable stock, whereas the V51 will feel very natural with it.
I just ordered an A2 stock for my V51P (I have a Fleming sear now) and will be getting my retractable stock converted for use with it. The retractable stock works ok on my HK91s but doesn't feel nearly as good as the fixed A2 stock when shooting. Even the Choate folder feels better (more solid and comfortable) than the A3 stock.