First and foremost is the fact that the Panama Canal was most attractive as a shipping route to the East Coast when Asia's manufacturing was strongest in places like Japan, Taiwan, and South Korea. As manufacturing in Asia has gradually moved to places like Indonesia, India, and Malaysia, the preferred route to the East Coast became the Suez Canal. Singapore is almost exactly equidistant from New York to the east and west, and therefore there would generally not be much of a difference in shipping costs via either route.
However, the Panama Canal route would have two major disadvantages versus the Suez Canal route even if there were no limit on the size of ships in either Canal:
1. The Suez is a "flat canal," while the Panama Canal contains a series of locks that reduce the operating efficiency of the canal system. As a result, transit times are faster through the Suez then the Panama.
2. The trans-Suez route to the East Coast a tremendous advantage over the Pacific/Panama route because a waterborne carrier can have its ships make port calls at two major consumer markets (Western Europe and the Eastern U.S.) instead of just one.
Something worth noting here is that when it comes to freight transportation to, from, and within the United States, all carriers and modes are in a "win-win" situation because the level of consumer activity here is growing so rapidly. What this means is that the Panama Canal route is not going to "take away" any shipping from the Suez route, or vice versa. Ocean-going traffic along both routes is going to continue to grow over time -- any improvements at either canal are simply going to make one of them grow faster than the other.