I'd go with a canister on the 55 gal
However, if you must go HOB
, then either an AquaClear or Penguin would do. Both have pros and both have cons. As a result, I actually run one of each on my 29 gal
. I have an AquaClear200 and a Penguin100. I like the Penguin for the BIO-wheel and deep penetrating intake (AquaClear gets deep, but not as deep). I like the AquaClear because it's quiet and creates more surface agitation (possible because it's 100gph more than the Penguin). I have the AC
set up to filter the top half of the tank and the Penguin to filter from the bottom and middle (middle via the middle intake on the tubes).
If you had to get one over the other, go with AquaClear.