Context...I live in Montclair and have lived in Hoboken and Manhattan.
Montclair is an awesome town. We love it. Awesome restaurants and shops. There are mini-celebs all over town (Colbert, Patrick Wilson, B. Brown, Brian Kenny from MLB). There are 6 train stations and Secaucus is an easy ride on Rt. 3. There are several new apartment complexes in town (with more popping up), making it easy for commuters. The restaurants are top notch and many are launched by former staff of top chefs in NY. The town is always full of people and always bumping til midnight. For the most part though, it is a dry town from a bars standpoint. There are only so many liquor licenses, so most restaurants are BYOB. That means bottle shops are all over town and couples often bring a bottle of wine to dinner. There are only 3 or 4 traditional bars (the rest associated with restaurants), making for a weak singles scene. It is a great family town, but no way would I live here in my 20s.
Hoboken is bumping and an awesome place for the younger crowd. Bars, restaurants, proximity to Manhattan, and for the most part a really attractive population. The Path train system is cheap and enables you to get into NYC very quickly. It is packed during the rush hour, but a quick ride. Hoboken is fun. It is full of young professionals, but is expensive.
If I were single and starting out though, Manhattan is the place to be. It was the best years of my life and the energy of living in NY is awesome. There are still some affordable locations in NY...some really old walkups and places a little further from the transit. I lived downtown in one of the business buildings converted to an apartment building - the rents have since skyrocketed down there, but I still work with several fresh analysts who live in NY on starting salaries. It is possible. If okay with roommates, it is possible to make it work.
As others have said, Nutley, West Orange (other side of Montclair, but without transit), Maplewood, Milburn, Livingston, etc. are all nice towns. However, they are family towns. If cost is really a concern, Bloomfield is an option with the new apartments very close to transit. The town is a little more blue collar, but is close to Montclair for entertainment and an easier commute to NYC via NJ Transit.