I have used both on two different horses. My WB is on CosequinASU and my Q/A is on SmartFlex. My WB did not do well on SmartFlex, but is very comfortable on CosequinASU. My 18 year old Q/A is very sound on SmartFlex and I saw no difference when he was on CosequinASU. So, try the SmartFlex first, if after a month no difference, go with CosequinASU. I also tried the CosequinSP, but didn’t like it as much as ASU. There is something about the ASU that really seems to help with joint issues. My WB has great hocks and he is 12 now.
Good Luck. Hope this helps.