You pay medicare and social security on entire profit in LLC.
LLC can choose S corp filing. Just do a google search on LLC vs S corp and you will read about the tax advantage of S corp election. In S corp you only pay social security and medicare on your salary while in LLC it's the entire profit.
my cpa informed me, LLC is not necessary for anyone not consistently earning over 50k profit annually from trading. The cost and complications exceed any tax savings. Plus it raises flags for audit. The benefits are for the people that sell LLC and corp setup, and cpa that charge extra for additional tax returns.