outofoptions:
He says he needs 600 feet and people keep bringing up breadboarding? I think we need to know what the projects are?
Note, he said he had 12 different projects, so assuming each project needs the same amount of wire, that might be 50 feet per project instead of 600. But even so, I would imagine at those differences, you have to be careful about the electronics side of things, and worry about errors, signal power levels, etc.
To the OP, it sounds like you are buying one big spool of wire. While it might work if it is the right size, if all of your wires are the same color, and there is some distance between the Arduino and the sensor/servo/led/etc., you likely will have a lot of problems debugging the thing to make sure the right wire is plugged into the right socket. My wife tells of her father, who was originally an electrician that worked in Con Edison in New York for a period. He got called to fix the wiring in a bar that evidently was one first places in the city to get power and still had original wiring. According to my wife this original wiring was before they started color coding the wires, and all of the wires were brown and her father had to figure out all of the connections, while the bar was still running. Now, maybe the story is true, and maybe it has been stretched in the retelling, but the point is if all of your wires are the same color, it may lead to problems down the road.
I should mention that some of the hookup wire I got from Radio Shack was too thick to be used in some breadboards. As others have said, you want solid wire that is 22 gauge.
So what are these projects that involve long distance wires?