Relative to each other...:
The Afuris have a lower base damage (14), firing rate (20.0), and reload speed (2.7 s), but also have a greater magazine size (70 rounds). The Afuris would most likely benefit from damage, reload speed, and fire rate mods.
The Twin Vipers have a lower magazine size (28 rounds) and reload speed (1.9 s), but also have a higher base damage (16) and firing rate (25.0). The Twin Vipers would greatly benefit from magazine size mods and probably reload speed ones as well (to offset the reload times).
Both of these weapons have the same accuracy (8.7), max ammo capacity (210), and critical damage multiplier values (150%). Plus, as you stated, both have a terrible rate of ammunition efficiency. They do have some armor penetration issues though, so keep that in mind.
Overall, the Twin Vipers have a greater base max damage potential (3,360) compared to the Afuris (2,940). Yet, the Afuris have a greater base damage per magazine (980) compared to the Twin Vipers (448). You'll be burning through each magazine on the Vipers much faster, necessitating more reloads. Do you need sustained and quick (burst) damage? Go for the Afuris. If you need long-term damage, go for the Twin Vipers. Personally, I say choose the Twin Vipers but, make sure to slap on some magazine size and reload speed mods.
Different void missions (extermination, mobile defence, etc) have different loot tables. Each time you finish a mission, one item from the loot tables is chosen as a reward by the RNG gods. A single void mission type's loot table consists always of many weapons' parts and usually, if not always, something else such as forma blueprint.
If you do another exterminate one, you might get another Dakra part, or you might get something else.
This outdated loot table should give you an idea.
Best Answer
While I have never witnessed drops disappearing, I have had a suspicion that they do, especially considering scenarios where you comb the entire map right before the end, and still end up having received considerably fewer drops than someone else in the squad. This would not happen if drops never decayed.
Also anecdotally, there seems to be a feeling in the community that they do indeed despawn.
I was not able to nail down an official source, but the thinking appears to be that there is a cap on the number of resources on the ground at one time. It would seem that after 50 items are on the ground, the 51st item will cause the 1st to despawn1.
From a programming perspective, this makes sense, as infinite new entities have a tendency to bog things down. This also explains why this phenomenon seems to be so hard to notice, as by the time you've let 50 other drops pile up you've probably completely forgotten about the first one. This also means that if the mission only drops <50 items, they do stick around forever.
1Of note, different categories of loot seem to have different "counters." Meaning health orbs, for example, won't count towards your maximum amount of displayed resources and so on.
2Also of note are these patch notes, which say
which implies quite strongly that other types of drops do expire.