You have absolutely nothing to gain by forming an entity with your circumstances and choice of products to trade. With a job, you can never claim this is your business. So you will never be able to turn capital gains into ordinary "earned income". With a S corp, you have nothing to gain either as everything will flow to your personal taxes. Just don't bother.
Hint: Find a trading entity and look up public records as to what is their org structure. They are not corps. They are LP or LLC. And as a single member trading stocks & ETFs, they get you nothing.
Disclaimer: IANAL
Actually, S Corp shareholders who are active in the business can pay themselves a "reasonable salary", and issue a W-2, and that is earned income.
http://www.entrepreneur.com/article/73368
Me and my business partner (who are unrelated) trade futures through an LLC, which are taxed as 60/40 long-term/short-term capital gains, so until the effective tax rate on our profits exceed 7.65% (our effective self-employment tax rate, considering we can deduct the employer's share of the taxes as a business expense), we won't be paying ourselves guaranteed payments (the equivalent of wages for LLC members, since they cannot also legally be employees and receive a W-2), we'll just take distributions.