Having used the Dr-5, DR-6 and the seagull right angle view finders I'm well aware of the pros and cons of them. The problem is there are two different viewfinder types on Nikons, the rectangular and the round, and this is where some work better than others as there is massive difference in their fitting.
rectangular viewfinders
For cameras like the d7100, d600, d750 etc there is not too much between the dr-6 and the seagull. The dr-6 slips on and off easily and is very to easy as it takes seconds to fit. The seagull has to use an adapter designed to fit the rectangular viewfinder and it works quite well. If the adapter is lost it won't fit, they are small and could be lost easily. The dr-6 is better quality build and slides straight on without adapters, but then it costs more.
Round viewfinders
Cameras like the d700, d800 use the dr-5 and the seagull if you purchase the extra 22mm round adapter. Of the two the dr-5 is much easier to use, you screw in the small adapter then the dr-5 onto that (note you can have the adapter attached to the dr-5 then screw both in together at once- it's much faster). To straighten the dr-5 you press the two side buttons in until it's where you want it and then it's locked in place. The seagull has a round adapter that is screwed onto the camera. There is a screwing nut on this that is used to hold the slotted adapter straight. The seagull adapter slides into the slot adapter part. The locking nut part is fiddly to get the slot straight. Also the round adapter on both the dr-5 and seagull are roughly the same and both are difficult to unscrew from the camera. Nikon supply a small spanner adapter with the dr-5 to unscrew it, seagull don't. My friend had to use a pair of pliers to get the seagull adapter off a d800, not something that's nice to do.
My preference is the dr-5 unless cost is an issue the seagull would be ok for rectangular viewfinders. The dr-5 is better for round viewfinders as it's much easier to use and faster to remove.