First; Wild Imps will be obviously more damage over that much time just based on the sheer number of Wild Imps you will summon vs the number of Dreadstalkers you cast (You can
at most get 48 total Dreadstalkers, not casts but total number of them, throughout a 6 minute fight VS
hundreds of Wild Imps).
Second; When you compare casts/summons you can't compare something that costs 2 Soul Shards to something you are casting 4 Soul Shards for, unfortunately that's not really a fair comparison since you cannot cast Dreadstalkers with more than 2 Soul Shards at any given point. In terms of DPET (Damage Per Execute Time), Dreadstalkers blow Imps out of the water.
This spreadsheet by Asyntax compares the damage of some pet abilities, what you want to look at there are the sections for Wild Imps and Dreadstalkers (the damage they deal is what you're comparing, since it's 1 Wild Imp vs 1 Dreadstalker there). Using the first character for sake of this example you can clearly see that over the course of their durations comparing 1 Wild Imp to 1 Dreadstalker you get something like this;
Wild Imp, 12s of Fel Firebolt: 3200 damage per cast (assuming 2s cast/12s duration = 6 casts) you get 19200 damage for 1 Soul Shard.
Dreadstalker, 12s of Autoattack (normalized attack speed of 1s + Dreadbite): 6900 damage per autoattack (1s auto attack/12s = 12 attacks) you get 82800 damage + 29600 damage from Dreadbite and you get 112400 damage for 1 Soul Shard.
So in terms of DPET (which can also be thought of for us, Damage per Soul Shard Spent) Dreadstalkers blow away Wild Imps without a shadow of a doubt (1 Dreadstalker is about 5.85x better than 1 Wild Imp).
Thirdly (just because I have it quoted); You go down the left side of the Artifact tree because they are all DPS traits compared to the right side which has some DPS trats and some non DPS traits and to maximize damage you go to the left since well....it's all damage.
TL;DR
Imps 'look' a lot better because you cast a TON more of them than you do Dreadstalkers.