Ok in NORMAL LoR(Law of Retribution), you can get any arcane from the LoR arcane reward pool. This includes the common, uncommon, and rare arcanes from LoR. The rarity is simply the chance of obtaining an arcane. So in normal LoR, you would more likely get common or uncommon arcanes than rare ones. However in Nightmare LoR, the arcane rewarded is always from the rare pool. So you will never get common or uncommon arcanes from the nightmare mode.

Now are rare arcanes better than common or uncommon ones. No. Look at Arcane Resistance. This is a bad arcane. It is also a rare one. The rarity has little to do with good arcanes or bad ones. Every arcane is a case by case bases if it is good or not. Like I said before, rarity is simply the chance of that arcane being dropped.

That being said, because nightmare mode always gives you a rare arcane, the common and uncommon arcanes in LoR might actually be harder to get, just because the item pool in normal LoR is that much bigger, hence the common or uncommon arcane you want to get will have a lower percent to drop.