You really need to talk to a LAWYER before you jump on the LLC or Corp bandwagon.
The BIGGEST mistake the layman makes in forming either is assuming that their assets are automatically protected. The fact is that a court can AND WILL declare your assets ripe for picking in a lawsuit if the plaintiff can prove that you manage the day to day operation of the business. The judge in the case will declare that your business is, in fact, a sole-proprietorship and make your assets available to liquidation in judgement.
The thing to protect your assets in that case isn't a LLC or Corp, but LIABILITY INSURANCE. When you purchase a policy, though, make sure it not only covers the investment directly involved, but the value of all your other assets as well.
[Edit] Forgot to add this little most commonly unknown fact about LLC's and Corps...
Think you might have to go to small claims court or municipal court to get a judgement against a tenant or file an eviction? In most states, you can't represent a LLC or Corp in any court unless you are an attorney. Expect to pay handsome fees for these simple filings in which a sole-proprietor can do himself.