For completeness starting from the ground up, i myself posted some of the recent models mentioned so far in qnapsys youtube page.
//Notes: some models may have been omitted in this thread, but i mentioned most of the most relevant models for most people. If you need fine tuned recommendations, check from the full list of models yourself
TS-x31P3 < TS-x51D < TS-x53D (x53be ???) < tvs-x72N , tvs-x72XT , ts-X77 series (this is the amd ryzen model)
There are a lot of models, it gets confusing. but here are some tips to figure out which to get
1. which cpu? arm? x86 Intel Celeron I5/I7 ? Amd Ryzen ???
2. how many bays? M.2 SSDs? 2.5'' SSDs? 3.5'' HDDs ? (i recommend 2 or 4bay 3.5'' hdd minimum)
3. expansion add-on card option? (things like additional 10gbe ports, M.2 SSD NVME support or other features)
4. support for expansion boxes ?
6. most recent release ( if a model gets replaced with a newer model, go for the newer one. span.com usually post the model release dates if you are unsure. and on forum they usually discuss if a model is replaced by another usually)
7. 1gbe? 2.5gbe? 5gbe? 10gbe? (these are native gbe ports on the NAS. but if your nas doesn't have them natively, they can be added on via a QM2 addon card, assuming the NAS model supports it, but this would require an additional purchase from native gbe port solutions)
While at it, for HDDs, for best bang for buck i'd probably recommend in 2020 is Seagate Ironwolf non pros due to the recent WD red NAS non pro drama You can see what other alternatives hdds people recommend in the same discussion thread here