The answer to this question depends on what the assumptions are. That's because there is more to cleaning a tank than just changing water. If the discussion is based just on water change alone, then the following link is helpful in comparing the efficiency between the two methods:
http://www.angelfish.net/DripSystemcalc.php
Here's some representative data from the above link for a 100 g tank (rounded to the nearest gallon).
10g drip results in 10g replaced
20g drip results in 18g replaced
35g drip results in 30g replaced
70g drip results in 50g replaced
95g drip results in 61g replaced
Thus if one is willing to accept a decrease in efficiency then the drip system can be utilized to accomplish the same benefit proving that other tank maintenance is also performed. There are also other considerations such as treating the new water.
A clean tank depends on more than just water change. It is also important to remove the uneaten food and feces at the bottom of the tank. The drip method does nothing to accomplish this while the displacement method accomplishes both at the same time (assuming its done in the proper manner). It is also important to perform other tasks such as wiping down the glass and cleaning other objects in the tank.
Assuming that a tank has an overflow pipe, the drip method can result in much less work. For example, suppose one has a 100g tank and it is desired to replace 30g daily. First remove the uneaten food and feces by syphoning 5g off the tank bottom. Then set up the drip method to place 35g of new water in the tank over a 24 hour period. This new water does not have to be preheated since the tank heater can do the job. Removing 5g of old water each day is considerably easier than handling and heating 30g each day.