The longer your chiller is, the more efficient it will be and the faster it's going to chill your wort.
If you already have the copper, the cost of an extra 25 feet isn't an issue. The only down side is that it will take a little more time to go that extra 25 feet. You can also loose any wort you leave in the chiller, so make sure you drain it all the way.
Obviously, a 50 foot chiller is going to take up more room than a 25 foot chiller.
Since you are in ON, your tap water should be cold enough in the summer, so it probably doesn't matter either way.
If you have a pump, you can always cycle your chilled wort back into your boil kettle for a few minutes until you get to your target temperature for fermenting, so the size of the chiller doesn't really matter if your willing to wait.