Mk3 & Next-Gen: what's the difference?

If as you say you are not interested in commercial streaming why bother with a streamer?
I understand that streaming CAN be playing music from a library (internal or external/NAS etc.) but IMO you are paying a lot for unused options.
You don’t mention your DAC but that, to me, is more likely an upgrade option than the box you use to store and push music to.
Is your DAC optimised more for USB or another input? Does your DAC only accept data files as passed to it or does it internally upsample and regenerate the data to another format before converting to music output (NOS, Ladder DAC or over sample/FPGA )?
You might be better off waiting for a couple of years to observe how Innuos and its competitors develop their product lines.
It's not very likely we will see an increase of products that exist only to play local files and have a reduction on streaming services, in fact the reverse is increasingly true. I do agree it's generally good to avoid investing in items that have vastly more features than you may ever need, but it perhaps does reach a point of false economy where you dismiss genuinely good options just because it has one or two things that are surplus to requirements. I never use coaxial inputs, but that doesn't mean I ignore all DACs that have this input. It's a bit like buying a new TV in this day and age; it's guaranteed to have things like Netflix built in, even if you never plan to use them and you just wanted something with great picture quality to watch your blu-rays.
 
It's not very likely we will see an increase of products that exist only to play local files and have a reduction on streaming services, in fact the reverse is increasingly true. I do agree it's generally good to avoid investing in items that have vastly more features than you may ever need, but it perhaps does reach a point of false economy where you dismiss genuinely good options just because it has one or two things that are surplus to requirements. I never use coaxial inputs, but that doesn't mean I ignore all DACs that have this input. It's a bit like buying a new TV in this day and age; it's guaranteed to have things like Netflix built in, even if you never plan to use them and you just wanted something with great picture quality to watch your
If someone is using less than 50% of the potential of a decent piece of equipment and he asks ‘is it worth upgrading’ then surely it is worth discussing the other major component(s) in his system that connect directly to it which also have a major contribution to the sonic performance of the system. Money might be more wisely spent there (or elsewhere ) before simply pointing to another Innuos model in the product range.
I see advances in the streaming world from Innuos and it’s competitors and waiting 6 months, a year or so for this customer (who acknowledges that his requirements NOW are minimalist) might have far greater benefits than jumping into another streamer box with lesser sonic improvements.
 
If someone is using less than 50% of the potential of a decent piece of equipment and he asks ‘is it worth upgrading’ then surely it is worth discussing the other major component(s) in his system that connect directly to it which also have a major contribution to the sonic performance of the system. Money might be more wisely spent there (or elsewhere ) before simply pointing to another Innuos model in the product range.
I see advances in the streaming world from Innuos and it’s competitors and waiting 6 months, a year or so for this customer (who acknowledges that his requirements NOW are minimalist) might have far greater benefits than jumping into another streamer box with lesser sonic improvements.
Certainly the whole system should be taken into consideration, but keep in mind as an existing Innuos customer the overall software suite is exactly the same if upgrading - current ZEN is 3.3.4, and so would be a STREAM/Next-Gen replacement. So if we are just talking hardware upgrade for sound...