sponsored or not is the difference
nobody really knows how these rules are applied to physicians that serve a specific commitment in time. however, in general the difference is:
EB2-with LC- is psonsored by an employer, so the basis for the GC is that you will take the job that you were certified for, once you get a GC. 6 mnths is the minimum period (colloqially) that you have to work before you can "change your mind (and job)".
If you move under AC21, that link is ended. You just need a 'similar job".
EB2 NIW is self petitioned. It ahs a specific commitment period- 5 years. Once that is complete- you have no link to the employer and the employer has no link to your GC petition.