Suitcases are a recurring motif in the book; when Holden meets the nuns, he has just checked his own suitcases into a luggage hold at the station. He feels it's sad that people like the nuns only have cheap suitcases to carry around all their belongings; also he himself has expensive suitcases, Gladstones.
He recalls his roommate Dick Slagle, who he liked because he's 'intelligent and... (has).. a good sense of humor' much like Holden himself. Dick also has inexpensive suitcases. Slagle labels Holden's more expensive possesssions 'bourgeois', a difficult word implying disdain for Holden's belongings and his supposed materialism. However, Slagle is secretly jealous of Holden's suitcases because he wants people to think the suitcases are his.
Holden tries to make his point when he says, 'it's really hard to be roommates with people if your suitcases are much better than theirs'. The point is about money and background; Holden can have more expensive suitcases than Dick because he comes from a more wealthy background. He himself feels that money is not important, and it makes him sad that it gets in the way of human relationships.