Sadly, the answer here is “it depends”. The factors it depends on will vary with the domain and how the owning servers are set up as well as how your own local DNS is set up.
First, for example, regarding the NS records returned: it is perfectly allowed to randomise the order in which those records are returned, so the order may differ each time you request it. On the other hand, that is not done by all DNS implementations, so you might well get a statically ordered list. The point is that you cannot be sure.
Next, some DNS implementations will query each NS in parallel, and use whichever one replies first. Others will hit each, determine the fastest over some number of requests and use that one. Or it could just round-robin.
There are multiple RFCs for DNS, two of the more useful that I have found are:
http://www.faqs.org/rfcs/rfc1912.html
http://www.faqs.org/rfcs/rfc1033.html
I realize this is something of a non-answer, without anything definitive for you to take away, but given the above, the only true way you have to determine the behavior for a given domain is to test.