Location, location...!
The location of the apartment is great, close to everything in the old town. I do not think it could be better.
If other reasons did not exist, just by location, I would advise Karen's apartment.
When we arrived Karen was waiting for us and in a friendly and very professional manner she explained everything we had to know about using her apartment.
One last thing to note: do not be scared with the stairs, since there is no lift and the apartment is on the top floor. The climb is quite easy, as the steps are low, and the views over Edinburgh castle worth the effort...