I would probably go with the big rocker. Many play cats are moving to bigger tubes for running the hard stuff. There is only a negligible difference in weight between the models and the big rocker gives you a few more options for carrying gear and accommodating heavier paddling partners. I believe that the big rocker is also marginally longer and wider, but only by a few inches - probably not a big deal unless you have some specific super tight runs in mind.
I have the straight tube model - not because of any reason other than I got a good deal on it. If I were buying new I would have gotten a big rocker - but at 6'5" and 210 lbs my size is a driving factor. Personally I also like having my feet lower, so a bigger tube creates a taller seat, and makes me feel more balanced - but again my size affects this feeling.
One final comment - the Shredder brochure I have lists the carrying capacity for the normal rocker as 400 lbs (sadly it doesn't list capacities for the other models). If you weight 175, plus 10 pounds of PFD, helmet, and paddling clothes your 185. Add another person at that size for 370. Than add in a rescue/pin/first aid/pump/patch materials for about 20 lbs. Now you are at 390 before any other gear - like a cooler for beer or 30-40 lbs per person for an overnighter. With 10 lbs of beer and 60 lbs of camping gear you're 60lbs over the weight recommendation for the regular rocker.