From my (admittedly limited) experience, blockade is more of an extortion method; i.e you pressgang passing ships into giving you dosh. Whereas patrol is actively seeking ships i.e looking for a confrontation.
On faction planets they really don't like you doing that, so they send someone to whup your ass. Not sure about indys.
As for which one is better really depends on what you hope to accomplish. Blockading some poor indy world isn't going to net you that much, but as you said the indys don't really throw any nasties at you. Whereas patrolling will net you various military/merchant ships to kill/loot/scrap which is significantly more dosh at a greater risk.