Not to be nitpiky but SWMO is actually in the Neosho watershed.
But to try and answer your question, even though both systems, the Elk and Shoal, are very close in proximity, they are different in many ways.
The Elk traverses a more rocky, hilly type topography more closely associated with those streams from the Ozark interior. The Elk and especially its tribs have a higher gradient drop per mile and a lesser amount nutrient density. In fact one trib has one of the steepest drops per mile in the state.
Shoal traverses a more transitional area from Ozark type topography to a more pairie type. It's just a little slower and with more nutrient density. It's gradient is a little less, although there are short areas of quick decent.
Shoal has more and better habitat than Elk. And I can't help but wonder what Elk would be like without all the traffic. Does the traffic cause an undue stress on the biodiversity of the stream?
I'm not and never have been a big fan of the Elk. I do love her tribs though.
Be very proud of your 18's from this corner. I've been fishing here all my life and have only caught one. The MDC has only found one 19 and very, very few 18. I have caught numerous 20's LMB from these rivers.
One thing to remember is that fish of these sizes are getting very near the end of there life span. And they don't get this big by being stupid. But to me that is part of what makes it so fun. The chase!