Top overhaul means you get a new set of pistons or just change the piston ring and cleaning of your valves and inspection of the crankshafts condition.
Full overhaul means opening up your whole engine, getting new bearing, new gasket, new piston rings, maybe even pistons, pulley, springs etc etc.
It depends on your engine condition itself, a full overhaul will cost you way more then a top overhaul, if your engine is running fine and its eating some black oil, then just get your piston rings replaced. Do not waste much money on overhauling, better go for transplant as it will cost less and you can ge a slightly more powerfull engine.
