We have 11 acres, but only two horses at the moment. However, we inherited an unbelievably large manure pile, from the previous owner, when we moved in. And, so I must agree with cowboydoc, that sufficient acreage/horse is important.
Regarding the straw: my wife and I have used straw at other points in time, but the amount of waste is probably 10 times the amount produced using wood shavings (we use shaving rakes to sift the shavings, as opposed to pitch forks).
So, if you decide to keep your manure and spread it yourself, I'd imagine wood shavings would be more manageable and require less work in the stall, than would straw.
In addition, we move our manure from the temporary pile, outside the barn, to a larger, more permanent pile that allows us to constantly turn the compost and therefore speed up decomposition of the wood.
hess