I tried one on yesterday; I have a BB58 black.
Very similar watches obviously ... just a bit smaller.
Probably for me the 54 would better suit my slim wrists but the difference isn't enough to make it worth selling the 58 and buying the 54.
Longer term I think the 58 will prove more popular as the 54 is small for this style of watch and would probably be only of interest to slim people.