The lower the better provided you are using a quality spiral wound wire. The best wires I have tested are the MSD wires at 30-40 Ohms per foot ($$$ though). The worst wires I have tested are used OE wires @15k+. Ohms per foot. The major difference between the 2 types of wire is that OE wires will wear out electrically. Over time the carbon thats coating the fiberglass on an OE wire will burn off leaving just fiberglass to act as a conductor. The spiral wound wires have no such carbon or any other substance to burn off therefor their resistance will change very minimally if any at all. The only possible down side to using a lower resistance wire is that you can potentially get a little AM radio noise but thats pretty unlikely unless your running an MSD or Jacobs Type ignition. The resistance measurement thing kinda goes out the window when you start into off brand junk that may use less than ideal materials even on a spiral wire.

I remember back when I was younger doing work on my vehicles. I would hook up a timing light and there where times that it was very difficult to get an inductive light to fire. Back then I thought it may be the vehicle missing or something but I soon figured it out. If your plug wire has too much resistance (older used OE wire) then you will not get a large enough pulse of current to trigger the inductive pickup on the timing light. Its pretty simple really, plug wires do 1 thing and thats conduct electricity. You need to buy what ever wire does that the best.