I have the 300M for now, they give a soft ride, but at the expense of durability. I guess if you don't mind replacing them once a year they are pretty good. What I ended up with were also the smallest in his line, it's what Junior recommended for a Calmini bumper and winch.
From my experience SLR is not a reputable company, I would recommend against ordering anything from them. Spencer Low Jr. is no better than a snake oil salesman.