I just used the Widow in a mission, and I like it more than the Javelin. Basically a "bolt action" sniper rifle, it packs a punch. Like the Javelin, it shoots through cover and pierces enemies, allowing a well placed shot to decimate a number of weaker opponents. The Widow, on the other hand, has no need for charge-up before it fires. Whether this is a blessing or a curse, I'll leave to personal preference (I like using the extra charge-up time to correct my aim). Stats-wise, as I said in my previous comment, they're near identical, with the Widow sporting a minute amount of extra damage, and immediate fire.
If weight seems to be a problem for you, the Mantis is still a very capable weapon. Similar stats to the last two, except less weight means less damage. It also can not pierce cover, or enemies based on my experimentation, so I still prefer the heavy-weight sniper-rifles.
If weight seems to be a problem for you, the Mantis is still a very capable weapon. Similar stats to the last two, except less weight means less damage. It also can not pierce cover, or enemies based on my experimentation, so I still prefer the heavy-weight sniper-rifles.