For purchases made within one visit, the two are the same so this is irrelevant. But for ones with multiple visits, which one to analyse? First Click or Last Click?
First Click can be argued as the true customer acquisition, where your business attracts the customers' interest. As such, it makes sense to optimise the most productive channels based on First Click analysis if such figures are available and reliable. Problem is, most Analytics softwares out there (including Google Analytics, WebTrends, Omniture -please correct me if I'm wrong-) report Last Click figures by default. Some funky solution may be needed to generate the First Click figures
Last Click, on the other hand, can be argued as the one serving the conversion points to the highly-interested customers. In other words, this is the one actually closing the deal, and without this you may not get as much sales as you are getting.
Is it true about the Last Click argument above? It may be worth spending a few hours to ponder about this according to your business.
But it is fair to say both are as important, and the main learning is "in optimising your leads through First Click analysis, make sure you do not unnecessarily pull your money from the most productive Last Click channel