Just tried both today in whistler.
Fs in a 181: Immediately noticed the difference in weight reduction. FS always light this years, and coming from PEs, they were as weightless as the burnt paper thats on the graphics (which are pretty sick, loads of hologram naked chicks too). In the park they had good pop and spun super easy. On the groomers they tracked nicely but were just a tad sketch cos they're so light. Ruled the bumps- rapid edge t edge. Sweet switch.
BC in 178: Loved em. Felt really planted despite being 3cm shorter. Not too heavy and good pop for their size -98mm waist! Hold an edge like a bitch when hooning but less maneuvrable and generally a bit harder work. Contest-winner graphics are ok. Sweet switch and they stomp landings.
I'm 6,1 and bout 185 and that's probably why the FS felt almost toooo light. Gonna definately buy some of the 178 BCs for jibs though, they killed it.