This morning I saw that de.wikipedia.org writes that the short key length of DES was chosen deliberately because already in the 70s the NSA had sufficient computational power to break this cipher ("Es gibt die Vermutung, dass diese kleine Schlüssellänge absichtlich gewählt wurde, weil die NSA bereits in den 1970er-Jahren genug Rechnerkapazität besaß, um diese Verschlüsselung zu brechen."). Oddly, there is no citation.
Thus, perhaps the AES S-box is less "secure" (whatever that means exactly) than the DES S-box but that doesn't really matter because AES has such long keys.
The S-boxes of DES, OTOH, had to be crafted in such an elaborate way that no attacks other than brute force are possible given the current state-of-the-art. This then would allow the NSA with their exceptional computing power to break the cipher by exhaustive search if they really needed to but nobody else could possibly pull that off (at the time!). Any other backdoor, based on some deliberately (or accidentally) injected weakness would be likely to be found by other smart people after some time and would have resulted in a practically feasible attack due to the small key length and hence small safety margin of DES.
The existing attacks on AES (as yet not completely confirmed) on the other hand, even if they shave off 20 or so bits of key size do not result in any practical weakness.