Touchscreens equipped with friction modulation can provide rich tactile feedback to their users. To date, there are no standard metrics to properly quantify the benefit brought by haptic feedback on touchscreen usability. The definition of such metrics is not straightforward since friction modulation technologies can be achieved by either ultrasonic waves or with electroadhesion. In addition, the output depends strongly on the user, both because of the mechanical behavior of the fingertip and personal tactile somatosensory capabilities. We investigate here a method to evaluate and compare the performance of haptic tablets on an objective scale. The method first defines some metrics using physical measurements of friction and latency. The comparison is completed with metrics based on pointing tasks performed by users. We evaluated the comparison method with two haptic devices, one based on ultrasonic friction modulation (Tpad) and the other based on electroadhesion (Tanvas).