I've intensively rided with the 151 while in Thailand (the board was just off the factory) and to be honest it is a killer board for light wind.. I'm very used to RSC boards, I ride them mostly since then I started kiteboarding but my RSC boards are not the square type of todays but more the surf style ones.. I'm not a fan of Square board at all but to be honest since 151 was much of a surprise, very very smooth, no aggressive at all and cool on your leg, but that's not only a light wind war machine, it kills also when the wind pop up..
I've ridden the 139 this week end in a good 30/35 kts and it was exactly the same board as the 151, yep altough the shape is different, the felling is exactly the same.. a very very good board for big wind !!
I'm not usualy a very enthousiastic man about boards, I love my RSC old school boards but I've to say that I will have to push some people to have those interstate added to my quiver eh eh eh