I'd guess the sheepskin would be better for shock absorption than the gel.
The Horse had an article
about a study done in Austria comparing gel, leather, foam, and reindeer fur pads. Only the reindeer fur pad significantly decreased the force on the horse's back. I've never heard of a reindeer fur pad (I guess they're common in Austria?), but from google image searches, it looks pretty similar to sheepskin.
Thinline is also a good choice, and is what I got for use under my saddle when my saddle fitter recommended something additional for shock absorption. Thinline occasionally sells their "slightly irregular" pads on eBay
for about 1/2 price- I got two from them last time they had a bunch up, and I couldn't really tell anything was "wrong" with it except that the white one was cut slightly smaller than the black one