This depends for me
We have a Greenway where I live. Depending on where you're going, taking the Greenway instead of 10th St will cut a good 5 min off your ride. However, that's rare unless you're going downtown. But on the other hand, the Greenway is a dang fine ride if the weather is nice, and I much prefer riding along the banks of the Tar River than playing Human Frogger with the jerkwad motorists in this town. As far as when I'm on the road, this varies too. There's a little spot where it's my favorite place in all of the town to bike... but I have to be in the mood for it. Yesterday, I needed to blow some steam and wasn't in a particular rush... took Cotanche St instead of my regular route. Never would have done that if I had somewhere to be.
So I suppose short answer, find a route that is ROUTE-ine for you, that's the quickest, most expeditious way for you to get there. Then find two more routes. One that's for a really nice day and you're in no hurry, and one for rain. If you take the same route, there's no harm in that, but variety spices something up (I forget what it was).