Short and simple: base damage > dps. Elly's explanation tells why.

And to clear up the confusion, your base damage is the main determinant for skill damage. In the OP's example, weapon a would have a greater impact on skill damage. This is why, IMO, the Sanguine Longbow is overpriced. Though it does have greater dps, the fractional increase in skill damage and significantly shorter range compared to the crossbow makes it overrated.