I just installed an aftermarket Kenwood this past weekend on the same system. I used the PAC-TATO one and all went smoothly. I even bought their other adapter (PAC-SWI I think)..which lets you retain the use of the steering wheel audio controls. They did a good job of showing you how to connect both units together (which I'd expect since they made both). It was a bit more involved having both adapters to hook up in tandem but still pretty straight forward. So, if you're only hooking up the TATO, it'll be a piece of cake.
I'm not sure about the Metra...but the TATO has dip switches on the side (where you see the red area in the pic) that let you turn up the gain if needed to +7db. I installed mine first with it at 0db...but definitely needed to switch it to +7db for the volume to work better with the Kenwood HU.