Having faced the same situation, here's my 2 cents:

• OMV is the best solution for reusing/upcycling old consumer grade PC hardware. Your storage pool is easily expandable using MergerFS, you don't need 16+ GB of RAM, and you certainly don't need server-grade hardware. But you won't have the bells and whistles the ZFS offers (yes, there is ZFS plugin, but at this point, why not just use TrueNAS?).
• TrueNAS if you intend to build a "serious" storage server with many GB-s of ECC RAM, multi-Gbit networking and all that jazz. And if you have the budget to buy 5 or 6 large HDD-s at once to start out your storage pool with a single vdev using RAIDz1 or RAIDz2 (or buy 2 HDDs for a single mirrored vdev with a whopping 50% of all your current and future storage going to redundancy). As I understand it, ZFS expandability is in the works, but not production-ready yet—which makes ZFS less suitable for ad hoc grow-it-as-you-go storage solution.

In the end, OMV won it out for me, the 10TB motley crew of various HDD-s has served me well and I can expand cheaply when my needs grow.