For an "RPG" they don't have much variety and creativity with items/drops. I wish they had Nirnroots!
All the weapons are the same no matter what rarity or ego with the exception of a few minor differences. The more rare the weapon, the higher the chance for a built in synergy. Also, as your weapon skill increases for a specific weapon type, all weapons in that catagory get permanent buffs. You can see what those are on your stat screen. Most of the power on the weapons is going to come from the specific mods you put on, what synergies are active, and what buffs you've gotten for that type.
Just remember that damage isn't the most important thing on a weapon all the time. Sawn off shotties have I think 1919 dmg, where as the saw is 400. The shottie only holds two rounds that get fired in unison, whereas the saw holds 85 and is fully auto. Not to mention the range difference. Different stats for different situations.