^ Since i personally own two of the species that your considering i would have to say i do like the look of my congo more than my palembangs, not to mention my Congo is far more active than my palembangs, although that could be just mine...
Really depends on what sort of puffer your looking for... but i have to say, the Palembang once fully grown will fill out your 55G a little better than the congo, although i'm sure your congo (if you go that route) will absolutely love the space... even though they are lurkers...
At the end of the day though, it all really just depends on which puffer you personally prefer more...