I use both, depending on what I can get my hands on. I prefer redwood, but there's no scientific basis for that preference. It stems solely from the fact that the redwood I've used is high-end redwood (tight-grained heartwood), and the cedar I've used is bottom-feeder fence slats. Naturally, I've got a much higher "twist" factor with the cedar that I rip on the table saw as a result. Enough to drive me to buy the more expensive redwood? Nope. I cut up the extra stuff and plop it next to a siding as mine props. Both hold up equally well in my experience.
I use both cedar and redwood, both heart cut.
And even some hemlock.
For now my buildings are treated and then stained with a dark stain.
Maybe in the future I will stain or paint some other than dark.
Will you westerners please send some redwood east? Most of that good stuff is not readily available in the east. I do have some in the shed and use it very sparingly. A friend gave me some redwood sticks. I cut some angle shapes for window frames.
Generally redwood weathers to a brown color. Cedar weathers to a grey color. Both may develope some dark or black areas. Both would do very well with an oil based stain.