In addition to all that Samuel said, the Barnes' arrangement of art can interest all levels of art appreciation. A child can sit in the room and try to find "all" the similarities between the pieces selected for that room (the triangular shape of the lady's hat in the painting is like the one in the ancient door hinge, the vase, the woodcarving... O, and the mountains and such in the other paintings.) The museum has the feel of relaxing in your rich uncle's sitting room until he returns. The Philadelphia Art Museum didn't start with the same history nor ever have the same intention. The Rodin is also a great place, located a short walk towards the Art Museum.
Well opinions and taste are highly individual. Both are excellent museums, and you might find one better-suited to your individual preference.
If I had to guess, there could be a few reasons for this. One is simply that the collection is amazing. It will impress anyone who goes. Additionally, this museum is smaller so it feels more intimate than the Art Museum. It is set up in the way it was in Barnes' house. This really adds to the experience. Also, it is newer so people are probably still excited about it. Either way, both are fantastic and worth a visit.
The Rodin collection within its walls is impressive, although considering above the Philly Art Museum can be a biased affirmation. Both venues have their own very important offers. Plus one is almost in front of the other.
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