A: For a small service business with no outside investors, the LLC form probably offers you more advantages than the corporation. (For example, LLCs offer greater flexibility, fewer corporate formalities, and pass-through taxation which is especially beneficial in the early stages of an LLC if you have other income sources.)
The S-Corp election does not affect your choice of entity (LLC or corporation). Either kind of entity can make the S-Corp election, which just tells the IRS how you should be taxed.
You can form an LLC at the beginning and make the S-Corp election for that LLC in the future, if and when it makes sense (you will still be an LLC then, but an S-Corp for tax purposes only). The S-Corp election only makes sense to do once your net profits reach a certain level. The purpose of the S-Corp election is to reduce your taxes. Filing the S-Corp election is not itself very expensive, but you will need to incur the costs of running payroll. (LLCs don't have to run payroll.)