Honestly, the current model of Frontier is pretty solid, and other than resale and aftermarket support, I don't think the Tacoma brings anything to the table over the Frontier. You will pay more for the Tacoma (which is why I own an Xterra instead of a 4runner). I loved my Tacoma, and planned to stay with Toyota, but when I started looking at newer 4runners, they were just more than I could dish out. Unless you are used to driving souped up diesels or sports cars, the Nissan 4.0 has more than enough power. At the end of the day you can't go wrong with either IMO.

As for the new Colorado, those things are huge, you may as well buy another 1/2 ton.