Probably pretty similar in the amount of protection.
The iSkin Solo is made from TPU which does a good job of absorbing shock and impact. And since it's one piece I think it'll stand a better chance of helping your phone survive a drop.
The problem with the Incase Slider seems to be with screen shields. It's bubbled up almost everyone one I've seen so far (only seen one person's who it didn't.) So if you a screen shield user that might rule the Slider out.
Neither is going to do a great job if the phone drops and lands screen side down.
Strictly on the criteria of which will do a better job of protecting your phone during a fall, I'd say the Solo.