While I don't go to coffee shops, I have tried both of their store bought coffees. I have to say, the Dunkin Donuts I had was a sweet flavored coffee, and it was terrible. My favorite out of either company in this form would be the Starbucks offerings. Specifically, their Sumatra blend that they make k-cups of.
I really think that both companies are overrated when it comes to their coffee. I happen to enjoy dark coffees without anything added to them. Both Starbucks and Dunkin Donuts don't have the bold and rich flavors I'm looking for on a consistent basis.
If I had to drink one coffee I'd probably stick with Folgers and their Black Silk coffee. For a cheap ground coffee, this is the way to go. The flavor is very good without adding anything else, and I happen to like my coffee without anything added. The flavor is better than a lot of the more expensive coffees, and it's still okay for my wife to mix flavors into with creamers or whatever she likes.
Have you tried making coffee with some of the Girl Scout Cookie creamers? I tried these a few months back and they were pretty good for a sugary addition to the morning!
1 - http://www.amazon.com/Starbucks-Sumatra-Coffee-K-Cups-count/dp/B0088JM82S
2 - http://www.folgerscoffee.com/coffees/black-silk-coffee