The floor choice you make should be based on a number of different factors...
1) What specific use is the shed/coop intended for long term.... if it is never meant to be anything other than a chicken coop then wood plank or plywood laid over 2x4 or 2x6 framing on 16" or 24" centers is fine and will last for many, many years.... if you intend to house other types of animals (goats/llama/ponies) then you begin to have issues with keeping the floor drained because the urine will begin to rot a wood floor in short order unless it is sloped well or there is a lot of bedding on it.
2) If you aren't sure that it is always going to be a coop and may end up using it for other storage then weight support may become an issue. Chickens don't weigh anything, so the above listed wood floor is fine. Garden tractors, ATVs or even other lawn mowers start adding the weight quickly, so stronger flooring should be considered if you want to 'multi task' the building.
3) Predators... depending on predators in your area you may want to either elevate a coop or reinforce a floor to avoid burrowing predators from getting in. We have a lot of skunks and weasels in our 'neck of the woods', black snakes are rather common too, so we chose to elevate our coop about 1.5 ft off of the ground and on the underside of the floor frame we attached metal sheeting we had taken off of a trailer we tore apart. Cement floor would be a good alternative to the need to elevate, though as others suggest it should be built with a good slope to a center drain for cleaning. You will probably find many critters may want to burrow under the coop and set up residence, but they probably won't be able to invade the coop itself, especially if you have a 6" + floor and run some reinforcement wire in it.
4) Remember, even if you place your flooring directly onto the ground, the ground can't really be considered as part of the 'support system' so to speak. I have seen numerous sheds which were placed direct onto dirt which after a few years developed very soft floors, lack of air flow can promote rot and the other problem is that critters that dig under for a cozy home remove large amounts of dirt and can actually leave floor joists without support under them (which allows them to become soft or sag over time) Sheds I've built to sit on ground level I place over a gravel base and build on some runners to elevate them a couple of inches to reduce moisture build up.
Sorry so long, but the base of your building can make all the difference in the world if you intend to use it for many years to come. Talk to some builders in your area, or better yet a few farmers, and see what problems they've run into with small buildings, it should help you decide how much you want to invest and how you feel best about doing it. If it is just to be used for a few seasons and then replaced then it's not so important... hope this didn't cause more confusion...