Of those I would head either to the Nautilus or the Regent. With the Regent not likely I'd push for the Nautilus. I don't know yet, other than being LATCHable as a booster, what the Frontier offers over a Nautilus. I have yet to see one in person. The Nautilus is nearly half the price of the Regent, has slots that are plenty high, and then converts to both a high back and a low back booster. It's also very easy to install and use.
You're right, the Go has top slots about the same as the Marathon. It also has no shell and requires a top tether at all times. When it converts to a booster at 60 pounds/17" torso it's only a backless booster. I think it's a great seat for travel or grandma's or something like that, but I don't think it would make a great daily seat. With no shell the only side protection that is offered is what comes with the five point harness. You say you have another child, so I would guess your son is sitting next to the window. There would be nothing between him and the glass in a Go other than the harness's protection. I'd be far more comfortable with a shell.
ETA: I see Jools has added some of the benefits of the Frontier. With two young children and a third on the way a nine year seat may be wonderful for you, the cost of the Frontier would be about $30 a year, or $2.50 a month. If you need a three across situation in the future the Regent simply will not work, though the Nautilus and Frontier should.
Wendy