Tell me about it ..picking a board ..geez ..yes a little bit more unstable ,they seem to roll over a bit more ,but you get a lot more looseness ..
the placid is 31.5 wide ,but its loose ..and doesn't grab in chop ,like my sharp railed boards ,doesn't feel as bitey on the wave face ,but you have the ability to turn at anytime ,where i find my 8,7 flow has a little more direction in turns and once you find your line ,its still good .the rails are sharper .and on a really clean face its good to have that too ..you can stay half way up the face of the wave easier ,where the rounded rails on the placid you can go anywhere ..I found walking up the nose a little bit will engage the rail a bit more if you are tube riding .
jumping from one board to another you have to surf them different ,
when i jump on the flow i have to cut across the wave a bit faster ,where the rounded rails i can drop straight down and not worry to much about the rail grabbing ,it just makes them easier to surf and you can get away with mistakes ,eg..late drops ,being too far up on the nose on a turn ,, the flash was the same ..quite rounded rails ,both amazed me what i was getting away with ..now i just push it to the limits .where you do notice it the most is your cut backs ..so much nicer with rounded rails .and you have the ability to change direction a lot quicker .
if your thinking of a custom ,maybe go to 30,25 .if your worried ,you won't notice it .
my 9,11 insane is 29 5/8 ,very rounded rails ..its unstable for me ,lucky it has the length ,which makes it easier to ride, .

i remember my 8,10 flow ,quite sharp rails ..one direction ....straight ..you had to wait for it to slow down before you could turn it