It is hard for me to recommend one over the other. They both are fairly comparable with each having a few points here and there in their favor.
So, I will just try to mention what I have noticed about each knife.

I "used" to have one of Justin's RD6 knives. I had a satin blade with cocobola scales.
The scales were of pretty wood, but they didn't line up too well (* I could have fixed them). But, they were contoured on the one I had, whereas they usually are just flat slabs with radiused corners. The handle felt pretty decent in the hand. I purchased the cocobola one on impulse from a Knifeforums listing. It looked nice in the picture.

But, I think you have to realize with Ranger knives in particular that they are made to be users and not collectible. They are not finished to high standards. So, I would choose micarta over cocobola or most other woods for durability.
I would also recommend the coating on Justin's knives instead of satin. I think Justin's 5160 needs all the rust prevention it can get and you will probably be happier with the look of the blade as the coating helps hide the rough grinds.
Since Justin's knives are rough (IMO - or at least the ones I have seen ..???...), one good thing about it is that you just kind of feel more relaxed about beating on it and grinding on it to make it your own.

The satin finish on my RD6 was pretty good, but there were some parts of the grind lines that didn't come to the top and weren't even.

I have a Scrap Yard S6 LE (satin finish). I don't generally like coatings especially on smaller knives. I sometimes tolerate them on larger knives. The S6 is right in the middle, but it is just rust resistant enough for me that I prefer the satin. A little Ren Wax will keep it good enough.
The grind lines and finish on the S6 are very good. The S6 is a knife that should be viewed as a user - Same as the RD6. But, I still like my knives to be decent looking. I generally consider micarta to be much better looking. But, micarta handles can be very comfortable or not depending on how it is shaped. With the (larger) Res-C handles, I know how they feel and I like them. There seems to be a large percentage of people who believe they feel nice.

I personally think the Res-C is much more comfortable in the hand than Justin's handles. But, I have noticed Justin's handles seem to vary in shape here and there. I think he tweeks the shape and blade shapes from time to time to suit what he feels needs to be changed. So maybe you will get one that feels good.

With Justin's micarta handles (from what I have seen and handled) and because of how the knives are made and used, I personally would be prepared to take my RD6 or RD whatever to my belt grinder to shape and contour the handle to suit my preferences. Luckily, the basic shape of Justin's handle is very condusive to being tweeked to fit well. All this opinion about Justin's handles is just my opinion. Others may feel differently.

The S6 does have a very thick blade, but it is a full height grind and doesn't seem out of proportion. The S6 actually balances well. I personally like the Scrap Yard S6 WAY more than the Swamp Rat M6. The Swamp Rat M6 just feels way too heavy in the handle to me. However, I don't think the S6 is large enough for me to call it a "Chopper". I would say the same for Justin's RD-6.

The 6" - 6.25" blades are just not a choppers in my book. You can chop some twigs and such and they will batton just fine, but not choppers to me.

To me, the S6 and RD6 are mid-sized multi-function knives.
I would say the S6 and RD6 are more of a large utility/field type knife than they are a small chopper - if that makes sense.

Both of these knives should be more than sufficiently tough.

Both knives can rust, but Justin's 5160 will rust if you look at it wrong. SR-77 will at least be fine for reasonably long periods in an air conditioned home. More maintenance is required for SR-77 in the field, moist environments and long term storage.

I have personally had better luck at getting and keeping 5160 sharp, but SR-77 can be made sharp.

The RD6 doesn't seem to be popping up to much on the secondary market lately, but the prices I have seen have been pretty reasonable and pretty close to the original issue prices. So, you can still probably make a reasonable purchase of a S6 if you can find one.

Justin's knives don't seem to come up on the secondary market much at all. I guess the majority of the people who purchase Justin's knifes just use them one way or another. They are good users.

But, with either Scrap Yard or Ranger knives, I am sure you could resale on the secondary market for about what you purchase it for.

So, I would recommend purchasing both so you can handle each in hand. If you don't like one, you can just turn around and sell it on Bladeforums, Knifeforums or ebay without much loss in money or time. This way may cost you $5 - $15 or you may actually make a profit (???), but you have a better way of getting what you want.

Good luck.