flashdash - 9:29 am on Sep 27, 2011 (gmt 0)
Not sure about SERP impact, but I remember Cutt answering the same question with something along the lines of "the site needs to be hosted in the same country as it targeted visitors". I can't find the link, even if I did, not sure it would still hold true as it was 2+ years ago.
My advise, go with your targeted visitors. If the site is global, look into analytics and see who are the most visitors with conversions, not just total visitors.
My 0.02 cents