Well by some amazing twist of fate I've got a 171 FCII (19.5cm waist), a 175 Behle GS (23 cm waist), and a 180 Blade Medium (23.5cm waist) so I think I can comment on this with some authority. I lack any comparison to other boards-I haven't ridden anything else so take that into consideration. I'm 195 pounds, have a size 30 boot, and ride in Wisconsin, 300-500 foot vertical hills with 2-3 trips out west each year, should get in a little more than 500,000 feet for the season.
The FCII is the easiest for me to ride and requires the least speed to get a carved turn going. I can crank the tightest turn on it too, especially at lower speeds. The GS board will carve a tight turn too if I'm going a little faster-it seems like it takes more speed to get it to bend. The Blade will turn tightly too but it's got to be tipped way up on it's edge. Once it's up there it bends really nicely. The GS board feels the most damp but the Blade feels really damp too. The FC is more lively. The wider boards (23 &23.5) are easier to balance on than the narrow one (19.5cm) when going slowly or getting on and off the lift but they don't feel more or less stable when going down the hill. I don't have a problem running different angles on the different boards, either way seems fine, but I guess this is a big deal to some people. I can't tell a difference in edge hold between the three of them! The Blade feels different when carving...like there's more (gripage) between your feet or something. I don't have any experience going off the groom with any of them. I think the FC is a little more versatile but even though they are different, I'm darn happy to ride any one. I think for really laying it over the Blade gives a more stable platform between your feet but you can get way over on the other boards too. I've got a case of "EC Fever" now so getting laid out is a high priority in my life LOL!
I'm interested to hear other comments too, I think they are both good choices...maybe the 170 Blade would be the best? a little tighter radius than the 180 but still pretty wide and soft so it'll bend really nicely into the arc. Good luck :)