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.
Shoulder bars, but with a petition for introducing the honor chain in the Imperial Navy uniform.