The Upper East Side has always been synonymous with wealth, while the Upper West Side has always been considered more bohemian and intellectual, probably due to its close proximity to Columbia University.
In the last 25 years, however, the Upper West Side has gotten progressively more and more gentrified, and now rent and condo prices on the Upper West Side are quite often even more expensive than on the Upper East Side. However, the Upper East Side is still generally considered the wealthiest neighborhood in the city.