Both will do the job well.
It very much comes down to price and personal taste
Alpex 4 k users tend to like that it looks like a scope, is lower above the rifle bore and definitely has better low light colour performance (due to it's larger objective lens)
Against that it doesn't come with scope rings or an IR illuminator and adjustable illuminator mount - and it's base price is £250 more than a Zulus
Despite it's poorer low light colour performance, the difference between the Zulus and Alpex 4 k in full darkness with IR is minimal
The Zulus is much lighter, and ready to go straight out of the box - a picatinny mount and excellent IR are already there
Both scopes have LRFs and ballistic calculators already fitted and ready to go as soon as the user has familiarised themselves with operation of the scopes
Cheers
Bruce