Dave Paine
When I last did some framing, I used my 20oz Estwing claw hammer with smooth face.

I like the thin handle of the Estwing, especially when I miss the head of the nail.

The nylon handle does a good job of dampening the vibration from the impact.

I also have a 28 oz waffle head framing hammer, which was only used for driving spikes. Too heavy for frequent use.

If your Habitat project is anything like the one I volunteered for, the materials were, let's just say, not what I would choose.

The lumber is likely to be low density pine, so a 20oz hammer is likely to be sufficient to drive the nails.