Since a smoke detector is generally placed on or near the ceiling (where smoke accumulates) and a CO detector is generally placed at or below eye level, it seems that at least one of the detectors in a combo unit would be in non-optimal location.
I don't think there's a big difference between the various CO units, though. All CO detectors are required to meet certain sensitivity standards (alarm must sound after X level of CO detected for Y minutes, etc). Get the cheapest unit. That way you can afford to buy twice as many.