Personally, I think the shoulder bars fit the theme of the imperial uniforms better, but the epaulettes have much more of a dramatic flair. Though if I had to pick, I'd vote shoulder bars.
Image source is wraithdt on Deviantart and the second image I found on a random forum, but is clearly an edit of the original.
[Image description: Thrawn in the Grand Admiral's uniform, on the left with gold epaulettes, on the right with gold shoulder bars.
Hey, Krennic's cape looked cool.... Dammit, what does that say about me.