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.
Unfortunately, as of Update 10.2 or 10.3, the drop tables have been encrypted. So, you cannot determine specific percentages of drops. However, as the Warframe wiki has indicated, each boss has a unique item which can be only dropped by them. Moreover, each of three factions has a tendency to drop certain mods and crafting materials over others.
Hopefully, this issue will be alleviated when the codex gets released...
EDIT:
I think there was an earlier version of the drop table released by creative crackers around a few weeks ago, but, it probably has become outdated with the recent patches.
Best Answer
The Reflex Guard mod is obtained as a rare drop from Corrupted MOAs. They can also be found as rewards from Tier 1 Void Survival. The Berserker mod is obtained as a rare drop from Corrupted Ancient Healers, or can gotten through transmutation.
You can use Nekro's Desecrate skill on these two types of enemies once they die, in order to have a second chance at receiving one of these mods. Personally, I suggest you do void survival, defense, or exterminate missions for the best shot at getting these mods.