Hay?...before and after

I'm interested in some facts about different types of hay. I have always thought that a good quality grass hay, fed free choice, is a reasonably good option for most horses. Since I moved to NJ it seems all I hear is timothy hay is much better for them. When I ask why, I hear various general reasons, nothing real specific. I'd like to hear what some of you know about the differences between grass and timothy hay (both good quality) as far as nutrition in a feeding program. One other question...this involves the end results of the hay. Most folks around here use shavings for bedding, then many of them spread the manure/shavings on their land. Doesn't this "sour" the land by altering the pH of it? I have a hard time finding a good supply of straw and if shavings are okay to spread without having to go back and "fix" any problems they may cause, it would give me an option when I need bedding.