The choice inside the big diameter family is between ten inches and twenty, and the diameter, which is the part that matters most for flow, is identical in both. So the decision is not about performance at a given moment but about how often you service it, how much space you have, and how heavy a full sump you want to handle.
What the shorter one gives up
Capacity, roughly by half, so on the same water the cartridge is changed about twice as often. For sediment duty that is the whole difference, since removal at a given rating is much the same in either length. For carbon the shortfall is more consequential: chlorine removal depends on contact time, which is bed volume divided by flow, so the shorter housing gives half the contact and correspondingly less complete removal at household flow.
What it keeps
The diameter, and therefore the low pressure drop that made the big blue family the right answer for a whole house feed in the first place. Media area scales with the square of the radius and only linearly with length, so the wide short cartridge still outperforms any slim housing comfortably on flow. It also costs less per cartridge, is far easier to handle when full, and fits brackets and spaces the long version cannot reach.
When the short one is the right call
Whenever clearance decides it, which is more often than people expect. The cartridge withdraws downwards and needs at least the sump depth in clear space below the housing at the actual mounting height, and under a low bench, in a crawl space or above a floor mounted appliance that space does not exist for the twenty inch version. It is also the better choice where one person has to service it alone, since a full long sump is genuinely heavy and awkward.
The arrangement that often beats both
Two ten inch housings in series, staged coarse then fine, will usually outlast a single twenty inch housing on dirty water, because the cheap coarse cartridge absorbs the bulk and the fine one lasts several times longer than it would alone. Two short housings also fit in places a single long one does not. Where carbon is the duty rather than sediment, length wins and the twenty inch is the better buy. Our whole house filter guide covers staged layouts.
Questions people ask about 10 x 4.5 water filter
Does the shorter housing restrict flow more?
Barely. Pressure drop is governed mainly by diameter, which is identical, so the difference is capacity rather than flow.
How much more often will I change the cartridge?
About twice as often on the same water, since the shorter cartridge holds roughly half the media.
Does it matter more for carbon than sediment?
Yes. Chlorine removal depends on contact time, so half the length gives noticeably less complete removal at household flow.
Is one long housing better than two short ones?
For carbon, yes. For sediment on dirty water, two staged coarse then fine usually outlast a single long housing.