As sabot loader said the difference is in screening Select has a minimum and a maximum screen while RS has only a maximum screen.
I ran comparison tests when the only other powder Black I did not find much difference in the three guns I used [all traditional] with #11 caps.
With #11 caps the ignition was more dependable as I had a few rare hang fires with Select. This was with 50 shots with each powder with each gun to me if you don't find a difference in that amount of shooting then there is not one worth looking for.
It is possible that a difference might show up with inlines and 209 ignition but I am not interested enough to do any more testing since I only use it in my Traditionals now. Blackhorn 209 is better suited to inlines in my opinion.