Best answer: Unless you specifically need a smaller SFX power supply because you're building a small form factor PC, you will find much more choice with ATX in both hardware and price.
ATX vs. SFX: It's all in the size
The important point to remember is that these two terms relate to a size of power supply, not a spec. One isn't necessarily better than the other.
And that's physical size, too, not capacity. SFX power supplies are physically smaller units than ATX power supplies, and that's the main reason you'd want one.
An increasing number of small form factor PC cases simply don't have the space required for a full ATX power supply. That's where SFX comes in.
ATX is the most likely choice
It's possible to get confused when considering an ATX power supply if you're also using a Micro-ATX or Mini-ITX motherboard in your PC. The size of the motherboard will always be described in your case. But ATX power supplies don't only apply to ATX cases.
In fact, you're more than likely going to need an ATX power supply. As above, unless your case specifically states that you need an SFX power supply, then you'll be OK with a normal ATX one.
Upgrading your PC and reusing the SFX power supply
If you need an SFX power supply now but in the future need to upgrade to a bigger PC case, you can re-use it. As the SFX size is smaller than ATX, you'll have no trouble fitting it in physically.
But, since they occupy very different footprints, you may need an adapter to properly mount an SFX power supply where normally you would use an ATX. It'll vary on a case-by-case basis, but don't just assume you'll have to throw it out.
If you're in the market for either an ATX or SFX power supply, these are currently our top recommendations for each.
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