If you designate a player and put him on waivers a team that claims him would have to pay his salary, if you release him then a claiming team would only pay the ML min while you pay the remainder of his salary. Best to designate with waivers so you'll open up a slot on the 40 man and possibly recoup his salary. If he's underproducing you have a shot, if it was a bad signing you're screwed.