Some general terms: ( there is a wiki article, but that's written before roboports
https://forums.factorio.com/wiki/inde ... ch_case%3F )
- belts are most useable for constant needs of very big amounts of items in an area up to 500 tiles (but can work much longer). Perfect for the furnaces, or for the basic items like wheels, elec. circuits...
Well... Basic belts are much cheaper than express and can't be beaten by tracks, when keeping also in mind, how complicated tracks are. And it's easy to put them in parallel. I made some time ago, where I had 5 belts in parallel from 1000 tiles away. This can transport about 2000 items/minute. Which is enough for nearly anything. But I can't recommend to play it like so, it's just no fun, until you are a transport-belt-engineer and like to optimize the capacity.
- logistic bots are suitable for more complex stuff. After you created the first bunch of blue potion to reach logistics, it's an option to rebuild all with logistics. They are perfect to bring the basic items to some assemblies, which produce the higher stuff. Or they are the perfect train station. I think, that they have limits, I cannot recommend networks with more than 500 bots. It's better to have some networks than one big.
- trains are perfect to transport many items over long distances. Long is 200 tiles (2 radar sizes). But keep in mind, that you can use the already laying tracks to make a longer track, this is much more difficult with belts and is the biggest advantage of railway. When keeping this in mind, the logic tells you to begin as soon as possible with trains.
But there are some more reasons, than just the pure numbers:
trains are pure fun. 
I constructed currently a network, which needs about three minutes to drive once around. Without stops. And once you constructed a train station with logistic bots that feels like... Hm... If you ever have built on a model railway, than this is it! You just stand there and watch and watch.
