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.