This is a good question and always pops up in many projects. We always try to look at the customer life time value as the long term KPI. In most businesses 20 % of your valuable customers account for 80 % of the overall sales. It helps to identify those customers, by setting up a clear funnel analytics over a longer period of time and measure each conversion step.
: from visit to registration, to sales, to repurchase and so forth. Also take into account the value of each sale. A customer that had to register to purchase, has shown a bigger commitment and might become more valuable in the long run. Many times it is quite easy to get registrations, what counts is the sales. On the other hand with registrations you have a valuable database you can retarget potential customers. I guess you should try, analyse the results and continuously optimize. Moreover this also depends on your kind of product and the traction you want pursue.