I would chose to purchase of NXDN and DMR for ZIP Code radios like the SDS or x36 type scanners over the 996 as you might use them while traveling in the ZIP Code or GPS mode and come cross one of these modes.
The 996 would tend to be used more locally so NXDN or DMR would be needed pretty much only of your normal targets use those modes.
Only add if it you really plan to use it.
I monitor a few DMR systems and south of me ( just out of radio range) are 2 NXDN agencies. However for my usage i dont feel the need to spend money on that feature. once in the last few years have i traveled to one of those areas.
So if you plan on using it, buy the feature. have it in both units if you plan to use both scanners.
I bought my 436HP used, and it had the MotoTRBO/DMR upgrade already in place. I needed to replace my 396XT because I mostly needed P25 Phase II and EDACS ProVoice capability. The 436HP travels with me, but I don’t know that I would’ve found the DMR upgrade worth the money to get if it didn’t already have it when I bought it.
It all depends on what’s in use around you that you’d like to monitor or where you might travel if it goes with you. For example, while the idea of maximizing my coverage is appealing, I have not added the NXDN upgrade to my 436HP simply because I haven’t found a compelling need for it in my area.
Between the two scanners you mention, without any more information than you’ve provided, if you know you’d like to monitor NXDN and DMR systems, I’d suggest upgrading the 536HP, as I think you’d get more mileage out of it.