The Nissin Di622 is very good. The mark 1 does TTL, the mark 2 does i-TTL. I would personally go for the MK1 in your boat as the D5100 doesn't do Nikon Flash commander anyway so any off camera stuff would likely be done manually or with cables which you don't need i-TTL for.
The Nikon SB400 is also decent for on camera flash stuff.