Someone said the DSR has a different riding position/with than the DS - probably a typo, as the two are exactly the same in those respects. The DS/R are wider and taller than the S/R.
The difference b/w DSR and a '15 DS is the controller and the motor (IPM on some of the '16 models). There was also a recent recall on certain '15 models, including the DS, so check if "yours" is subject to that recall as you don't want to get it then have it spent weeks in the shop. Or maybe you do, if they lower the price...
Where do you live? I have a '15 SR and I've only got close to overheating it once - I was riding on 90F day as fast as I could pretty much, maintaining around 80 with bursts to near max speed often. And it took a few miles to get the warning light to come on, but I did not get to experience the power loss as I slowed down to normal speeds, in the 65-75mph.
I've never had am issue in my daily commute. You will only see it in sustained near 80mph on very hot days. If your commute includes that, get the '16 with IPM (not all '16 bikes have that motor, only the ones with the large battery).
As for R vs. non-R, that's up to you. I like the R much better and the non-R feels underpowered to me. If you rode the R in Sport mode and liked it, you should try the non-R too to see if it satisfies you. That's a lot of money so spend it wisely
Lastly, demo bikes have shorter warranty - check carefully with Zero. Their warranty starts not on the day you buy them, but on the day they got on the floor as a demo. Or something like that - mine is a demo and I did not get the full 2 year warranty but something closer to 1.5 years.