I received my Stripology ruler a couple of days ago and also tested it out against my June Tailor Shape Cut Plus. However, my conclusions are a little different.
Regarding slippage, it's true that the CGS does not slip. However, I found that if I place a Slidelock ruler on top of the Shape Cut, it also does not slip. (Actually, this probably works using any 6" wide ruler when placed on top of the Shape Cut.)
The CGS slots are so rigid, I had difficulty using my 60mm rotary cutter in them. Had to switch to my 45mm rotary cutter. I can easily use my 60mm rotary cutter with the June Tailor mat.
It's true that the CGS ruler is wider -- 20" vs 18" on the Shape Cut Plus. However, I think this is more than offset by the height of the CGS ruler. The slots on the June Tailor Shape Cut Plus are almost 2" taller than the CGS. What I noticed when cutting Kona muslin is that, after folding the fabric twice, the CGS wasn't quite tall enough to cut strips cleanly; I had to finish by cutting the selvedge on each strip. (When I fold, I do like to leave the selvedges peeking out about 1/4"; someone who folds exactly to the selvedge ends may not have my problem.)
I'm on the fence about the markings. I actually like the less cluttered look of the June Tailor Shape Cut Plus, but there could well be advantages to the additional markings on the CGS that I just haven't needed yet.
So, my preference is to use the June Tailor Shape Cut Plus with the Slidelock ruler on top. This works better for me so far, and I think it offers a lot of versatility. I don't think anyone needs both the JT Shape Cut Plus and the Stripology ruler so, for those who already have the Shape Cut Plus and/or other June Tailor mats, the large Slidelock ruler might be the way to go.