So many people just assume that you need to incorporate. Sure, your accountant would love if you got an LLC or S-corp, it's more work and $$ for him. But it's not a clear-cut decision.
There are a couple of distinct advantages that incorporating does have over being a sole prop: liability protection and perhaps it lends an air of legitimacy about your business. If you need these, then by all means, go ahead and incorporate.
Otherwise, from a pure tax standpoint, there's a misconception that there are more tax breaks available to corporations than to sole proprietors. There are not. The costs are different and you (or your accountant) need to look at your own circumstances to figure out which structure is better for you.
As an S-corp, as mentioned above, you still have to pay yourself a reasonable salary, which is subject to SE tax. However, social security, which comprises most of the SE tax, maxes out at about 100k or so. So if you're making a lot more than 100k, the SE tax is not as large of a hit. Bottom line: you do save some money as an S-corp, how much depends upon your income and what you're claiming as your salary.
On the downside: There are state-specific franchise taxes that you may need to pay. As an S corp in California, for example, you must pay the greater of $800 or 1% of your profit. That's every year. This is not something you need to pay if you're a sole proprietor.
Also, you obviously have a lot more paperwork, with additional withholding, etc. So factor in the actual cost of incorporating and all the additional ongoing paperwork.
Sooooo.....there is no simple answer.
Oh, one more add: you don't need to be an LLC or corp to get your own EIN. Any entity type can get one.