Shock replacement

So I bought a 2014 Challenger RT Shaker a couple months ago. I noticed at highway speeds on the great roads of PA that the back end seems to get unsettled when hitting any kind of surface imperfection. I chalked it up to crap roads, and large heavy car. 
 

I was doing some work under it yesterday and noticed the passenger side rear shock is covered in grime while the rest of the car is super clean. I'm thinking the seal blew out hence the grime. I did check the axle and brakes and no signs of issues. 
Car  came factory on bilsteins as it's a super track pack and car has 21,000 miles on it currently. 

So my question is can I replace both rear shocks with Koni SRT which I found for about the cost of a single bilstein shock, or replace just the one shock with a bilstein? I always replace shocks in pairs but 21,000 miles isn't much and they aren't cheap. 
 

In reply to rattfink81 :

I know some people love Bilsteins, but I'll always choose Koni instead if they're available. 

Pete Gossett (Forum Supporter) said:

In reply to rattfink81 :

I know some people love Bilsteins, but I'll always choose Koni instead if they're available. 

this. I've used both. Sometimes on the same car. At the same time. 

I never met a Koni I liked   Too much initial damping, makes for an unnecessarily jarring ride.  They "feel" sporty because of this.

Thanks, I'm going to pick up the pair of  koni's then. I use to run bilstein's sport shocks and HD's on my 80's euro-trash cars(e30's and mk1/Mk2 vw's) but I've heard a lot of complaints about them in the last 10 or so years. Figure I'll swap the fronts out after the winter as a set of cop car steelies and winter tires are the next big purchase. 

Pete, well I guess I'll give them a shot at $160 for the pair. It's a 4000 pound car that I daily less then 20 miles a day so they have to be better then a blown shock. 

Pete. (l33t FS) said:

I never met a Koni I liked   Too much initial damping, makes for an unnecessarily jarring ride.  They "feel" sporty because of this.

Funny, that's the complaint I usually hear about Bilsteins.

02Pilot said:

Pete. (l33t FS) said:

I never met a Koni I liked   Too much initial damping, makes for an unnecessarily jarring ride.  They "feel" sporty because of this.

Funny, that's the complaint I usually hear about Bilsteins.

Exactly. Though more specifically every Bilstein I've encountered had way too much compression damping. My experience was mostly limited to VAG produce - and I know a lot of those people love Bilstein, but they were both jarring on the street & lacking at autox, including a set of PSS3's(?) I installed on a friend's Jetta, then removed & installed on another friend's NB Turbo S.