Interesting question:

I actually think Rebecca was a better comic foil for Sam.

She was trying to run a bar that was his bar. Coming into the show late, we already sided with Sam and that helped generate mileage for their conflict.

There was also a wildcard quality to Rebecca, where anything could happen, that Diane didn't have.

But I think Diane and Sam were better diametrically opposed. She was sophisticated and smart. He was the handsome jock. A very fire and ice quality about their relationship that is great for comedy as well.

I actually think they did a great job of making Rebecca her own character and not just a cheap knock-off of Diane.

I may also think Rebecca is the better comic foil for Sam because it seemed most of the conflict was between her and Sam and no one else. She was pretty much 100% a foil for Sam.

Diane had rivalries with everyone. Carla, as a waitress (because Diane sucked). With Friaser because they used to be a couple. Not all "Diane" episodes had Sam as a foil.

As far as romantic entanglement -- Diane all the way.

Both were great characters.