We are using butterfly valves to throttle limestone and calcium sulphate slurry flow in 4" and 6" pipes, not the ideal application for this type of valves I realize, but hey, we simply spend a couple hundred dollars every few months to change the discs and it has worked out OK for us.  We are considering changing to, for example, pinch valves, but because of cost and the required piping changes we have not made the move yet.  The original butterfly valves had CPVC or PVC discs, so all replacement discs I have ordered have also been PVC or CPVC.  However, I've been told that if I switch to polypropilene discs we should get significantly more wear resistance
V-Ball valves are great for paper stock.  The ball is usually Cr-Plated 317 SS. 
On slurry, Once the stuff chews thru the Cr, the 317 is pretty soft. Especially if the ball valve is controlling at small flows-then the V-Ball becomes a keyhole-ball when the v-notch wears thru at the point of the V.  
AND, a ball valve costs many times more than an equivalent-size butterfly valve. 

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