The script will take care of everything. q values should not concern you; they will be variable for different runs and will depend on the project's complexity. Different sievers will be chosen by the script, too; their rate of sieving might seem sometimes logical to the untrained eye and sometimes unexpectedly slower (on the next project)  but you should trust the script.
Here are the fairly good rules of thumb:
 for SNFS, with every 9 additional digits of SNFS difficulty, the overall runtime will double (or, which is the same, runtime will be 10x for every 30 digits)
 for GNFS, with every 5 additional digits of length(N), the overall runtime will double
Remember just these two and you will be in good shape.
Examples: a 120digit SNFS complexity project might take 1 corehour on your computer. Then you can assume that a 150digit SNFS complexity project will take 10 hours, a 180digit SNFS complexity project will take 100 hours, and a 210digit SNFS complexity project will take 1000 hours (~40 coredays). Ok?
You can estimate how hard a GNFS project will be by taking length of the input number N, and converting to (N30)*1.8 = similarly hard SNFS complexity. For example, a 130digit GNFS is approximately as hard as a 180SNFSdigit difficulty SNFS (from the estimate above, ~100 corehours).
