what model GNU and how much do you weight?
you may feel a significant difference between different models 4 cm apart (you can feel the difference between different models at the same size), but the difference won't be that noticable between a 152 and 156 Sierrascope especially at 160 lbs, which is what the OP weighs... the 156 certainly won't feel like a boat compared to the 152....
gnu carbon credit, and i weigh 135 pounds.
i do realize that i bought the gnu a little larger than would normally be suitable for me, but they didn't have any 154's, and i thought the length would suit me for all-mountain riding, as i am completely incapable of doing anything in the park.
i've gotten a little better since then, and i just went riding for three days straight. aside from the difference in flex and edge feel, the length does make a noticeable difference for me. it may be because i don't weigh much, but yeah, i don't know what else to say. as far as the ride, they both ride very well, and i can control both boards without any problems. the major differences for me were that the sierrascope is a noodle compared to the gnu, and the gnu is much more stable at high speeds coming down steeper slopes.
as far as the difference between a 152 and a 156 sierrascope, i have no idea. it sounds to me, given the specs of the op, that 152 would seem appropriate.