It is probably a lot easier for a passenger to spot other planes. That's what I usually find
I think the hardest thing to spot would be another plane on a closing track from one side. You've got to be doing an awfully wide scan to see one of those.
TAS/TCAS and compulsory Mode C+ would be nice but all the statistical data shows that one could fly blindfolded (EN-ROUTE) and would still die of old age.