I do something I know y'all think is nuts, I use my best RP as a SuA and a "flawed" guy as my Closer.
Think back to when Mariano Rivera pitched the 8th inning and John Wetteland pitched the 9th. Everyone knew Mariano was better.
As has been said, I want my SuA available for the most important innings. Often the 9th inning isn't it. Three outs starting with no men on base is technically a low leverage spot. So I go for the ugly save. Think Fernando Rodney.
I use a Closer because he's a guy I don't mind taking that extra low leverage inning off my best reliever's hands - so my best reliever isn't gassed for the next game. My Closers are usually pitchers with low DUR/STA, maybe lower splits, sometimes even lesser control if the splits are good. My best closer ever was a Lefty with one pitch - his P1 was 81 and his P2 was 60. I always had another RP on the roster who was "better" than him, that guy got the 7th and 8th innings.