Really depends on how you have anomolies set. If you have abundant then you definately want survey capabile ships very early. (right after getting impulse engines) For me i found it was too easy to exploit anomilies by building 3-4 fast long range cargo survey ships, so i now set them to rare.
Now i only start building scout ships once the rush is over, and I need to know where more resources are that I don't have constructors on the way too....
I'm still comptemplating whats the better scout, more speed or more sensor ? Either way I tend to put 2-3 ex life supports on them. I think i'll do the math on what covers more ground faster...