I think the number one reason BY FAR is how the gun looks. But defenetly, usage is number two. If you are gonna be out in the woods a lot, you can easily scratch a beautiful woodjob, and it will defenelty hurt you more than scratching a synthetic finish. Synthetic is also considerably lighter, which means you will not get as tired in the field from carrying a gun, but it also means less mass, and thus, more recoil. Wood finish also tend to require more care if you want to maintain the top quality finish, wereas a synthetic scope you can only give the bare minimum of maintenace. Synthetics also tend to camouflage better (especially at night), as they are not so shiny as a finished wood stock (the glare from the stock can give you away in a hurry!). Other than that, I cant think of anything else...
Either way, if you plan to use the gun for hunting, I do strongly recommend that you buy and attach a sling. Trust me, carrying a gun around will wear you out with time, regardless if its the lighter synthetic or the heavier wood.