If its of any help there is another thread which asks about the future of these two choices, the short answer seems to be that Nomad pack / Nomad native cli will most likely become the preferred choice in the future: Future of Nomad pack and Levant - #6 by jrasell though it does also specify that the future is never certain so I don’t think there is a concrete answer for this right now at least that I know of 
Personally I am using Levant for all of my job file templating, however this is due to a combination of me building all my current deployment pipelines before Nomad pack was even released in tech preview and the lack of clarity on to what will be happening in the future. I’ve not run into any of the known issues with levant not supporting 1.3+ functionality listed here: Support latest Nomad 1.3.+ job descriptor · Issue #457 · hashicorp/levant · GitHub as I’m not using the built in Nomad service registry but your mileage may vary!
I’m looking forward to when the Nomad team will fully clarify whats going to upcomming as I share the same sentiments as @rwojsznis in their response Future of Nomad pack and Levant - #7 by rwojsznis mostly so I know when its a good time to rewrite all of my CI pipelines again
TL;DR
The answer is mostly likely Nomad pack and Nomad native CLI but its not certain right now as far as I know 
Hopefully someone else can also weigh in on this!