The answer is honestly neither. The Main downtown area is called Montclair Center which is a decent walk from Bay Street and Walnut Street. There is a "second" downtown on Walnut street right next to that train station.
The issue with Montclair and the Bay Street station is that the town stuck all of the affordable and low income housing near that station and it's near the "low rent" district of montclair so 1. It doesn't feel charming 2. It sometimes feels unsafe and 3. it's far from the main downtown that you refer to.
The downtown in Montclair starts at Lackawanna Plaza which used to be the old Train Station. Makes sense the downtown starts at the train station. Montclair moved the train station hundreds of yards more east closer to Glen Ridge (aka Bay Street Station) so it's a pretty decent walk past low income housing or up a "dead zone" along bloomfield avenue before you get to the downtown.
See below. Show the two train stations and the distance from the "main downtown"
Montclair Light Rail (Step 1) | Fix Montclair