Michael - the answer to your question can be a long one, and probably mostly off topic for this forum. I'll try to be brief.
Higher res scan = better image, even if Pshop re-resolves to lower PPI for web (Pshop might do a better job than the Epson SW). Then, later, when you want to submit to another show, gallery, etc, you can re-resolve again (without rescanning) to their requirements, if greater than APUG's.
I've never been on the DPUG site, but you can probably find out more there.
With more and more exhibitions, competitions, etc requiring digital submittals, it's becoming harder and harder to avoid at least a basic understanding of this. Like many, I have mixed feelings about this.