I own an sl and heritage in 155 and have rode the proto. The proto isn't really softer than the sl in practice - just far less damp.
The heritage, while stiffer than the sl is fine for park. It's not any good for presses but its better on jumps.
The heritage is a better 'all mt' board for me because it charges way harder than the sl. Way better for busting thru and over chop.