LTS vs. non-LTS .... unless you plan to buy support from Zentyal, I don't think debate is really on the Zentyal side (that is made of opensource components) but rather at OS level.
If your concern is only "security updates", you will get it from system update, including for components used to build Zentyal, except perhaps for Zentyal code itself but this one is, from my standpoint less prone to such kind of problem as this is
mainly presentation layer.
Given the huge amount of changes that are introduced with each new Zentyal release, I would rather focus on feature as decision driver rather than security updates. If your concerns is not security updates but
support (with an "S" like in LT
S ), then there is no debate, go for LTS.