which push or pull factor do you think made the biggest impact on people's decision to move out west?

Standard of living

Explanation:

PULL FACTORS are factors relating to the home country of the immigrants, while the PUSH FACTORS are factors relating to the point of destination of the immigrants.

For the PULL FACTOR, it is the POOR STANDARD OF LIVING in the immigrants' home country that influences their decision to move out west, and for the PUSH FACTOR, it is the promise of a GOOD STANDARD OF LIVING that impact on the immigrant's decided to move out west.

Hence, the PUSH or PULL FACTOR that made the biggest impact on people's decision to move out west is a STANDARD OF LIVING.