Modeling Long Ago
Advantages:
From 1900-1950 is a Halcyon period of steam engines, a variety of operations, shortlines, narrow gauge, passenger operations.
Disadvantages:
Do you like brown boxcars? Do you hate Helvetica and anything jet set? Do reliable diesels seem too easy to you? Do you like squinting at silver nitrate negatives to try and discern what color to paint something?
Modeling Your Youth
Advantages:
Those good old days when you had plenty of time to watch trains and no money to buy them.
Disadvantages:
Does it give you a concept you can work what you want into? Are you in the demographic that manufacturers are catering too?
Modeling the Present Day
Advantages:
It is what you are appreciating now. You can go take a nice color photograph of it, or better yet, someone else already has and posted it on the internet.
Disadvantages:
Limited switching and shortlines. Most trains 3+ engines and 100+ cars, difficult to model on a modest layout. Really easy to just spend your way into a layout.
Modeling Your Neighborhood
Advantages:
Easy to get materials
Disadvantages:
In your basement is the same as outside.
Modeling Far Away
Advantages:
Tropical port / city? Icy Alaskan tundra? South American mountainside?
Disadvantages:
May look nice, but have limited operation potential, and be harder to get accurate detailing of.
Modeling Idealized
Advantages:
Exactly what you want
Disadvantages:
May be hard to keep coherence when you keep piling things on.