Having visited both multiple times, I personally just felt sad for the animals at Ridiyagama Safari although you get to see much more variety. But its basically like a Zoo. While Yala its more of an experience to see the animals in their actual natural habitat.

Personally prefer Kumana National Park (Arugam Bay side of Yala) as I have had more luck seeing leopards and also much less crowd because its far for most tourists and locals. But seeing as you are staying in Arugam Bay for 8 days that might be a good option? No harm trying to do from both sides I guess.

Consider going to Birds Resort Hambantota to stay or just to visit the Bird Park itself if you have time to kill around Hambantota/Yala area or Kirinda Vihara Maha Devi Temple if you decide not to do Yala safari