We're kinda doing the relief ace thing by default right now though, because Papelbon is holding down the closer's role competently freeing up Bard to pitch high leverage late innings.
My position on closer versus relief ace is that if you're having the debate, it's because you have only one solid reliever and you'll never get anything meaningful done anyhow. You need great relievers in the 7th, 8th, and 9th inning roles regardless, whether they pitch those roles exclusively or are a more freewheeling bullpen by committee or relief ace model.. It really doesn't matter which role you fill first, ultimately you need to fill them all.
One thought to bear in mind though -- the Guardians used the relief ace model in 07 with an adequate-at-best closer type, and Joe Blow got them through the Yankees, but they fizzled and died against the Sox when their relief ace, Rafael Betancourt, fell apart. They didn't even trust Joe Blow in the 9th against the Sox, and when he did pitch in that ALCS, he got shelled. So it really doesn't matter in the end, if you have to put an incompetent reliever in either the closer or the setup role, he's still going to cost you. If you're leaning too hard on any one reliever, closer or otherwise, that guy is at risk to fizzle.