If you grill on a patio or don't like ashy messes, then get the OTG. The OTS doesn't do a great job of keeping the ashes off of the ground. The OTG keeps everything contained.
Way back in the 80s I had a Weber kettle. I have no idea what model. But the design was like the OTS with the bowl/pan/whatever to catch the ashes. It cooked GREAT, but I always hated the ash mess and the convoluted way I had to lift and empty that pan. After I got a gasser in 1998, SWMBO made me get rid of the kettle (I still miss it.) Then the gasser died earlier this year, and I got a OTG (26.75). And I really love the ash catcher arrangement. Such a convenience and well worth the $ in my opinion. In addition to the other features Noe mentions. Sorry for the long response. Reminiscing a little
Bought an "H" (1987) model OTS, handed it down to my son this spring when I took delivery of my new Performer. 23 long, long regretful years of not spending the extra cash for the OTG.
I picked up a new-in-box 22" OTG on CL for $40. The family apparently won it in a drawing and didn't need it, "because they already had a grill." I gladly paid them and smiled all the way home.
My patio is getting crowded; I may have to sell a OTS.