Rank up advise - 4* duped or 5* unduped SL
So as the title suggests, I have a 4* single duped SL, and a 5* unduped SL, at R4 and R2 respectively. I have 4 T4 tech, should I R5 my 4* SL, or should I R3 my 5* SL in hopes I will dupe him in the future, so he can be stronger when R4ed?