I've always been a Nortel guy....so I'd go for the DR5....
why?
-Nortel phones have clean distinct and simple labeling of the buttons....with clearly defined Hold, Release and Feature buttons.
-Punch down arrangement is easy and concise.
-Programming the phones and KSU are a breeze with a little practice.
-Overhead paging is a simple connection to the 66 block.
-Nortel voice quality on even analog sets is far superior to other systems.
-Nortel phones are rugged and can take a beating.
-Ring tones can be changed via a simple feature code.
-Nortel systems are everywhere....many large national chains have legacy Nortel DR5's installed all over the nation and even world. You'll have no problem finding easy support, even on online blogs like this one. This is a very, very popular system.
-If you ever want to updated to a BCM, you can reuse the phones. If they are T series sets, you can reuse the phones on an Avaya IP office.
Cons? I'll have to let an "anti-Nortel" person convince you the cons. I personally feel when I walk into a business and I see a Nortel system installed, the customer made a smart business move and chose a timeless phone system that is rugged and still has phones that look good 24 years after they were first introduced. (M series) sets. If you are hooking these sets up to an overhead paging system....the voice quality mimics that of a high end (with the right TOA or Bogen amp) microphone system in an auditorium.