Definitely not looking for a fight here but IMHO, relegating gph to "means absolutely nothing" is a bit of a stretch. Gph is often cited as a measuring stick for determining filtering requirements with 10x considered sufficient in all but the most unusual situations. I think a case could be made for gph being more important relative to mechanical filtration as opposed to chemical or biological but that doesn't make it meaningless.
IME, media capacity is a factor but not necessarily a deciding one. Some biological media are better than others but much of the bio media on the market offers more surface area than could ever be fully colonized. The majority of tanks simply do not produce enough ammonia and nitrites to support a bio colony that could take advantage of that much surface area.
I know some very experienced aquarists that use AquaClears exclusively on tanks large and small. ACs are dependable, quiet, inexpensive, very convenient to maintain, and come with a two year warranty which is the longest I'm aware of. Bottom line, they do the job and do it well. So do Fluvals, Renas, and Eheims. The choice between HOBs and canisters is more often one of personal preference in terms of the visibility of the filter and proximity of the back of the tank to the wall, than it is of media capacity or some perceived superior filtering capability.
The species of fish you plan to maintain is certainly a consideration when planning filters but again IMHO, only in the context of some species producing more waste and therefore benefitting from higher turnover rates.
Whatever you decide Floater, good luck with the new setup!!
