There's also a matter of starting speed. When pulling from blocks or rack you must accelerate the bar from zero. Compare that to full version where bar alredy has some speed from the first pull. Also, RDLs start with an eccentric so you can use the stored elastic energy etc.
In my opinion, SLDLs are are harder on the low back since you're supposed to go straight down. In RDL, the key motion is pushing the ass back while low back remains arched.
I think a good choice would be doing heavy conventional DLs one one day of the week, and then some lighter RDLs on another day.