They won't drop in missions unless its from a Challenging mode named Elite
https://www.reddit.com/r/thedivision/comments/4efyco/how_to_obtain_gear_sets_11_edition/
Big debate on this.
Strikes has a 1% damage increase per hit, but decreases 2% with a miss. It appears to be very situational. Get yourself something with a large mag (like an LMG) aim at a target that you can't miss, and you can stack your damage boost up high, then switch to another target and land a few high DPS shots before the buff starts to drop off.
However, using this buff with an SMG where you get lots of misses seems very ineffectual. But if you can get it to wprk, you can stack this buff across multiple group members and all get a damage boost.
The Senty's set however, gives a 15% headshot damage boost per bullet hit, up to a max of 3 bullets. Its essentially a debuff on the person you're shooting so you can't stack it across team members for a 135% increase like you can with strikers. But its much easier to obtain as you only have to hit the head 3 times and then everybody in your group gets a headshoot damage boost hitting that person.
TLDR Version:
Sentry is probably better because its easier to get and less punishing when you miss.