first off: if your current set up feels good for now keep it as is.

in my recent experience: i've ridden several moderate to long tours 100 - 500 miles. i commute via bike and also race occasionally. all my bikes have 170mm cranks. i recently tried 175s on my touring bike for a little extra torque and oomph. but i had left knee pain for the first time on a tour to montreal. i gave myself some time to acclimate to 175s but ultimately went back to 170s. my bike guru friends extoll the virtues of longer cranks for extra power but i prefer to spin at easier gears but with less wear and tear on my knees. i'm right about 6' with a 33" bike inseam.