For starters we are dealing with a human being, so its never as simple as pigeon holing someone. But putting that aside
Secondly it is about building teams.
The combination of having broad platform skills in one person and broad design skills in another is a fantastic combo to get to work togeth
Thirdly, I think design skills are about as hard to acquire as good platform skills.
fourthly, good platform skills people are actually really good at getting into new platforms if that is what the project is going to be using. They often like to get into a lot more of the nitty gritty than a generalist
So, the deciding factor for me would probablly what has each achieved, how complicated was it, and what kind of skills gaps do we have on the team. I'd expect both to have a willingness to learn new things.