I bought the 8r in black and the 12r in gaucho. I figured I'd try them both and send back the size/color I didn't want. The only problem was that my 12r's were fulfilled by amazon and shipped via prime, so I got them much earlier.  I tried them on with different outfits and tried to wait for the 8r's that were coming from another seller. Well, when I couldn't wait any longer I wore them out and realized this meant I was keeping my 12r's. When the 8r's came I knew I'd have to send them back ($$$, right?), but they were soo great.
So, long story short, I have 12r's in gaucho and 8r's in black and I LOVE THEM BOTH!  They are different enough, I think. I wear them with everything and they're super comfy. The 8r's are so short that you will see some wrinkle from even the skinniest of jeans above the boot. However, I even wear boyfriends and straight jeans w/ the 8's because they're short enough to pull off a loose/sloppy tuck-in. The 12's cover more jean, so the look is cleaner, but I can only tuck in skinny jeans or leggings or it looks silly. I can't tell you which is better- I love them both. If I did it over I think I'd buy frye harness 12r's in brown and engineer 8's in black just so they would be more different. However, I am not unhappy with having 2 pair of engineers because they are super awesome. I kind of want some more boots now...
Oh, also, I found them true to size and easy to put on/break in.