The main difference between an LLC and an S-Corp is the taxation of profit distributions to business owners/shareholders.
With an LLC, you must pay self-employment taxes on profit distributions (unless you meet certain loopholes). With an S-Corp, self-employment taxes are not imposed on the company profit distributions.
Let's say your company generates $500K revenue and has $300K in expenses (including your salary), resulting in a $200K profit. With an S-Corp, you (the company owner) receive this money as a distribution and have to pay income tax on it.
Well with an LLC, you receive the money as a distribution from the company and you have the pleasure to pay income tax and self-employment taxes. That is an extra 15.3% tax (the current self-employment tax rate) that you have to pay ($30,600).
There are some other very minor differences between the corporate structures detailed here:
http://bizfilings.com/learning/corptypes.asp#LLCvsS
* I am not a lawyer or accountant, so please make sure to confirm everything before making a decision.