Rice and Slaton both play in zone schemes that inflate their stats somewhat, but both are still tremendous talents.
Slaton is basically a Warrick Dunn clone in my eyes. If he gets in space, he'll probably take it to the house. Truthfully, his lack of contact is what makes him a question mark. He doesn't take many hits, so we don't know how durable he is. He does have a nagging wrist injury that's been a problem.
Rice, he's probably the best in DI-A at waiting for holes to develop. His game is consistency. Ray just keeps chugging away, 4 yards a pop every time, it's very rare to see him tackled for loss. In the fourth quarter, he actually gets stronger and eventually breaks one for a huge gain. Unfortunately, this also has lead to overuse. He's only 5'7 (listed in 5'9 on the roster), so that's another mark against him in the minds of pro scouts. He has 4.6 speed, but his initial burst is as fast as anyone. That's basically the opposite of Brian Leonard, who had 4.5 top end speed, but took a while to accelerate. Rice's other main drawback is that he gets a lot of carries, and he's never really gotten the chance to catch screen passes.
If you want to see the clearest evidence of why Rice's speed will be held against him, click here and go to 10 min in. This play has stood with me for months because it really evidences my concerns. Revis (4.4 speed) chases him down on what should be an easy TD.