[embee]

As the chain wears the pitch gets bigger (clearances in the pins/bushes increases). As the sprockets wear the tooth pitch hardly changes, or with the front if anything the pitch gets smaller as the root of the tooth profile where the roller seats wears and the effective diameter gets slightly smaller. The net result whatever condition is that the loading gets concentrated on the take-up tooth. A new tooth profile is designed to meet the chain roller at an ideal angle, a worn tooth means the contact happens at a different angle so results in greater impact, making it less refined.
The bigger problem is often the hooking and the drag which can happen as the chain leaves the tooth on exit.
I tend to work on the principle that front sprockets get the hardest time and are usually the least expensive, so fitting a new one roughly halfway through the chain life can be effective and certainly gives a better condition than a worn one. I know it isn't an ideal or perfect situation, but only a complete new set of bits will give you that.
Entirely up to the individual, I certainly wouldn't suggest either as necessarily better.