either one works fine if you already have a good relationship with a title company who will close the simultaneous
however…
if you do an assignment, the seller and the buyer know exactly what your profit is. If your profit is a lot, they could both back out of the deal.
With a simultaneous close, the seller and buyer do not know your profit and YOU do not need to be on title. You can take title in a land trust with an out of state trustee, so YOU are hidden.
or
You can set up an LLC then sell the LLC so the buyer is the LLC, just a different owner.
there is always a way when you have a motivated sellers, a willing buyer, and YOU who found a GREAT deal.