The most intuitive answer is to say "Global Economy"! "The Internet made as live in one global village so location doesn't matter anymore".
But life is a bit more complicated. You have mentioned that the guy supposed to be your CTO, or basically, your main developer and probably supervising other developers.In that case, some of the points you should consider are: will he need to work closely with others in the company? Will he need to supervise others? Will he be able to perform his duties remotely without supervision?
Don't forget that there is a time difference as well but this could be bridged and not an issue when you schedule a call for early-late afternoon in Europe when in the US we just starting our day. This can be an advantage as well as this way the "Company's working day" is extended.
Which brings me to the next question - Where in the US will he be? East Coast? West Coast? Else?
Will he be in a place where he can do more than programming but also business development and meet with relevant clients/investors/partners?
If you have more questions or want to give more information give me a call to discuss.