I haven't even worked with him because he's really short.

Scale him up. I use about 115% if he's going to be in a scene with Vicky.

But people say they like David better than M3, why? I hear he poses better than M3. Really?

Yes, IMO he poses better. And he morphs better. A lot of the M3 problems were fixed with David. You can bulk him up, and his muscles look very natural, instead of the weird look M3 gets. David's face is also more easily morphed. He's made to look like a teenager, but "ages up" even better than M3, IMO.

Can I scale David up to wear M3 clothes?

Not really. Well, you can, but only in the way you can scale the clothing of any figure to fit any other.

I think i heard there is a way to transfer head morphs between David and M3 too, anyone know how?

You could try Morph Manager. However, I like David's own head morphs better. I think you can inject V3's head morphs into David pretty easily. Steph's, too, probably.