TarpTents are lighter, and I'd forgotten about the room. In fact, if you are traveling alone, the extra room of something like the Rainbow (the only one I'm familiar with personally) might mean you wouldn't need a two-person tent.

However, as I recall, you mostly hike with your son? In that case, you'll definitely want the two person version, but you'll be amazed at the extra room.

I'd probably be using a Tarptent except for one issue that is specific to the area and season in which I hike: the Ohio River Valley, in the summer, is hot and humid. If you set up a tent with the rainfly, it focuses the heat and becomes stiflingly hot. With the MSR Hubba (or Carbon Reflex, a lighter version of the Hubba), I can pitch it without the fly and take advantage of all that mesh to avoid the buildup of heat. With the TarpTent (or other single-wall tent), the rainfly is always on and the heat builds up. (I spent a summer with a Rainbow, and couldn't solve that puzzle.) However, if I were hiking just about anywhere west of the Mississippi, I'd be using a Tarptent in a heartbeat. They're well-made, well-designed, and a great choice. Except in Ohio and Kentucky. In the summer.