I've myself written something like the first, but the second approach suits my needs better. typeof(this) is very nice.

It is. For mixin templates, it's a must.

I intend to use it (among other things) for multivectors, so the scalar type is not directly obtainable from the parts, but then, I can allow partwise multiplication with whatever type the parts may multiply.

Yes, that can be tested at compile-time.

The nicest thing about the two approaches is that they are orthogonal - a single template mixin may implement them both!