Vietnam is more diverse in both climate and geography. It has high mountains which are cool and wet, beach resorts , historic towns and Halong bay. Cambodia has Angkor and nice beach resorts too. For scenery and climate options Vietnam wins hands down , both are relatively cheap. Cambodia is nearer and therefore easier to get to, don't even have to fly. If it was me I would go Vietnam more places to visit, and cooler weather in the north.