If the saddle is basically a good fit but maybe a bit wide in front, a sheepskin pad is good.
If the horse has conformation issues such that no saddle is going to fit well, a shim pad is necessary.
If the saddle fits well, you donāt want or need a sheepskin pad because obviously it alters saddle fit.
The only time Iāve used a shim pad is for an older borrowed mare with a slightly sunken back and a borrowed saddle. But in that case for a saddle that bridged I couldnāt use either Thinline or Mattes because they only had 4 shim pockets.
My choice was LeMjieux or TSF 6 pocket sheepskin shim pads. I went with TSF.
If the horse had belonged to me I would have put the cash towards getting a saddle reflocked for her.
So really it depends on what is wrong with your saddle fit and why you canāt address that first.
If you expect to be dealing with multiple poorly fitting saddles in future a shim pad could be a useful investment