The choice between 170 and 175 depends maily on your leg length. If you are above abouy 6' tall, then 175 may better suited to your legs.
Work out the highest gear you currently need (in gear inches). There is no need to go higher. I have used a 30/40/50 and a 28/38/48. There are a few times I have wanted lower gears but I have never needed higher. Exactly what gearing you need depends on your strength, the terrain and your load.