To me Greyson looks like a first name and Grayson is a last name. So, yeah, I prefer the "e" with Grey and a nn.
I checked the SS baby names list and Grayson was 97th in 2011 and Greyson was 200th. So it's getting trendier, but not quite popular yet. But it's steadily moving up the list so in 2-3 years it could break into the top 50 and then be a trendy thing.