From my understanding, the lower the grain count (135-155) compared to 180, the less the 'snappiness' that some users report. So in essence, it really depends on the application. If I were looking for a SD round, I'd go with a lower grain, because it's suppose to fly faster, translating to high energy impact, and expands more then a heavier, slower bullet( read: 180 grain).
If you are looking for practice ammo, 180 should be fine. I read an article online that articulates what I told you slightly better, I'll try to find it for you.
Here is a list for comparison of the Muzzle energy of various rounds, i believe also including the two weights you asked about if not very similar rounds of the same brand and weight: