Quote from traderstatus:
The tax law on LLCs is flexible. Generally anything REASONABLE goes. If you are actively involved, full-time, doing the day-to-day operations of the Medical Staffing business then, IMO, reasonable might mean most or all of your income should be subject to self-employment taxation.
On the other hand, if your earnings come to you based more on the capital invested, rather than your personal services rendered, then the LLC might allow you to avoid some or all of those self-employment taxes..
Not so with a sole-proprietorship. A sole-proprietorship would result in 100% being taxable no matter if it was a result of services or capital...
I think it depends on the state that your LLC is registered. Usually as a manager of a LLC (unless it is managed by others), the owner has to pay SS tax (at least it is the case in MA).