Updated: Jan 30, 2019
It's a common question and one that isn't easy to answer Let us start by discussing the woods themselves and a wee bit of the history of use in guitars
Spruce Wood - 'Picea' Spruce wood has been used in the making of guitar tops for centuries, a lot of the makers from the 19th/20th century used Spruce for the guitar top. There is varying different Spruces that have been used in guitar making Spruce is an excellent wood in terms of workability/stability and most importantly sustainability One of the reasons it is commonly used in musical instrument production is because of it's flexibility while also remaining stiff, these properties are crucial for the way it vibrates and 'pumps' air around the cavity of the guitar. Aesthetically it is pale in colour, with very straight tight grain. It tends to age and mature well with the colour darkening to a yellowish hue and the sound will continue to develop for a long time.
Cedar Wood - 'Cedrela/Thuja'
Cedar wood has only been popularly used for the guitar top fairly recently having a spike in popularity in 1960 but has been used in guitar necks especially Classicals for a very long time. Like Spruce there is varying different Cedars that have been used in guitar making Some of these like Spanish Cedar are now protected species, thus making it hard to find and ship, needing Cites certificates. Because of this I would say it isn't quite as sustainable as Spruce, but the more common species like Western Red Cedar are very sustainable. It shares similar properties with Spruce in terms of workability/stability, it is much softer than Spruce so is quite prone to dents. I find it to be slightly more flexible than Spruce. It has a soft brown/reddish appearance with very tight and straight grain. Tonal Properties Well this is the hard part, there is a lot to what affects the tone of a guitar, from the internal structure, to the thickness of the top, even to how well the guitar is made or even the method of construction. Makers tend to say that a Cedar top will develop quicker whereas a Spruce top takes a while to mature. Spruce is considered to be more direct and brighter. the listener will be able to hear every note like it is being focused on them. Cedar is fuller sounding, with a warmer tone that is said to fill a room with sound. Apparently during blind tests a listener will struggle to distinguish between the two, which I don't find surprising. Conclusion
Well it's hard to say, I like to build with both and there is differences in tone but in general I believe that it purely personal preference. It could come down to playing, listening or aesthetics. Play both and work out what works best for you