I know you posted this a little while ago but I just saw it. I am an owner of the JPW culebra and have paddeled it through some big water (r2 of course). I also have the pleasure of having great friends that own shredders and I find myself opinionated about the issue. I have also paddled the shredder through big water as well. Here are my thoughts.
JPW Culebra: It has it's high points. The foot brace in the back is nice because you can wedge in tighter than you feel like you can in a shredder. The foot cups in the culebra leave a little something to be desired in my opinion. I am a smaller frame female and I usually paddle with my husband who is quite a bit larger and stronger than me, so the foot cup thing might be preference since I don't think he cares either way. The Culebra has larger diameter tubes , a plus in most cases, it sits higher out of the water, creating less drag and seems to turn a bit quicker. Don't get me wong the dshredder turns fast too. When (if) you do flip (which I have many times), the culebra has high thwarts and has a tendancy to want to right itself. Now if (when) you do swim becasue of the large tubes I found it really hard to get back in (we women generally lack upper body stength). We have added the "rescue" handles on the sides and a strap accross the thwarts and I can get it much easier now. The material is holding up exceptionally well and I have no complaints about that. The Culebra is "held together" by 4 d rings that you strap just below the inside of the thwarts, because of this design the culebra twists and tourques when one side in under more pressure in the water. I haven't figured out if this is a good thing or not.
My biggest complaint with the Culebra is the floor is laced onto the base of the raft, this creates a seam/gap in the front. As you punch through big rapids this gap/seam has a tendancy to hold you in the rapid and significantly slow down your momentum (this is mainly the reason for our swims). In general if you plan to only run class III or IV- you will be fine, otherwise the Shredder or the Hyside version are a better choice.
The shredder: Overall a great design and a class V capable boat (if the paddelers are class V). the part about a shredder that sucks is it is black. Black boats and hot weather or bright sun are not a good combination. Easy to pop this boat if you are not careful and burn you skin if you get on without a good rinsing or cold river water when it is sunny. Ihae had minor burns a couple times because I forgot about it being black.
Honestly if I had to do it over again I would buy a shredder.