To reduce the cost of trade, and shorten the time to move goods from one place to another.
Columbus
To get silk and spices
Spain did not set out to establish a new world. They were looking for a shorter route to a destination they already knew about. They tripped over the "New World" by mistake. They thought they were in the East Indies.
The change was that they was looking for a water route from Europe to indies people and their life was change
They were looking for a shorter route to India and China.They made landfall in the west indies in an attempt to find the western passage through to the east indies. It was an accident which became profitable.
Because they were looking for a shorter/better route to the Indies; thus the native Americans were called: Red Indians
Simply for the trade and the money it would bring.
Christopher Columbus was actually looking for a shorter route to Asia.
Spain was looking for an alternate route to the East Indies from the one that the Portuguese had charted. Columbus suggested such a possibility, so the Spanish Monarchy was willing to support him.
1492
A shorter sea route to India
He was looking for an alternative rout to the indies and the magnificent cities that Marco Polo spoke of.
He could have used either of the routes you suggest. However Columbus was on a journey of exploration to see if there was a shorter faster route to the Indies by sailing to the west.