The only difference between SHS and MTH track is that MTH used rail with a slightly different metallurgy. The difference when building a layout is immaterial. Fox Valley rail does not have pre-weathered webs so there is work to paint the rail web to match the SHS/MTH weathering.
In my experience 27"R is the minimum for scale operation with KaDee couplers unless all cars are 40' boxcars and all engines are short wheelbase like SW-1's and SW-9's. Using all #5 turnouts is fine unless you wander off into the world of long wheelbase River Raisin engines. I used a lot of #6 and #8 turnouts because I had the space and they look good. They were not necessary for reliable operation.
