Point well taken about the the lack of need for the optional mode upgrades. However, IMHO, at least here in the NYC area, there is enough DMR (nothwithstanding ham radio) traffic to keep the listener entertained for hours. I love listening to strings of expletives streaming over the channels as unwary users think that DMR somehow protects them from being heard. And there are some quasi-governmental (and perhaps even one or two "official" agencies, that use DMR in their communications activities.
I would suggest that you play with your SDS100 for several weeks to make sure it is working properly technically, and that it's a "keeper", since those upgrades are non-returnable and non-refundable. If you've got the extra bucks (usually $60 - $75), I would definitely go for the DMR.
Have fun with your new toy.
- Mike