That's a difficult question, because the forehand low serve and flick are less common nowadays, which means more deceptive because we are not used to see it as much (anticipation). But from a technical point of view the flick with a backhand is more deceptive because using the fingers is quicker and more powerful than using your wrist (forehand serve).
Well that's a lot of pros for the forehand high serve and actually I should start working on it. Yet I'll still add two extra advantages about working on low backhand serves: - On the longer term, It will also improve the overall footwork ability of the player - Some of those footwork exercises can easily be done alone and off-court through shadow drills such as the one I described in my previous post thus add an extra training session outside of the club schedule. Always beneficial.