Rackmount Power: NEMA 5 or IEC C14?

Here in the USA, most consumer power strips seem to make use of NEMA 5-20R connectors - they look something like this. However, in the rackmount/datacenter world, I see a lot of power distribution units that accept IEC C14 connections instead, which look like this.

I know there's no significant difference electrically; I'm wondering why I should prefer one over the other. In my limited experience, I've found that connecting C14 connections to a PDU results in looser connections that I'm more likely to have fall out without securing them with zipties or velcro.

I think it's a question of voltage. NEMA 5-20R can only support 125V (the common household voltage in the US...minus your electric clothes-dryer, of course). IEC C14 can support up to 240V, a more common voltage outside the US.

So why would 240V be better than 125V? The higher the voltage, the more efficient the power supply (I believe because it doesn't have to be stepped down as much from the building's electrical feel...which is 480V (or higher, with even larger buildings).

By way of a real-world example, we've switched one of our DataCenters to be entirely 3-phase 208V power (using IEC C13/14 and C19/20 outlets). We're expecting to lower our bill by almost 50% compared to the 125V power (not accounting for overall growth).

Guamaniac hit on the difference, but his numbers are slightly off.

If you are in a commercial building with 3-phase power, then you may be able to run your servers at 208V. (240V will not be available. That is only in residential 2-phase power. None of the racks I looked at were certified to run at 240V in North America.) Most power supplies in modern servers are auto-sensing, and will run more efficiently and cooler at the higher voltage.

Since you won't want people to come along and plug in devices that require 120V (also known as 115V), the electrical code requires using C14 instead of NEMA.

For our server room, I chose to go with 120V. Since my servers are generally lightly loaded, I wouldn't save much on electricity anyway. It was more important to be able to be able to plug in devices like monitors, KVM units, and small switches that we already owned, anywhere in the room without having to run double the number of circuits. So we just ran NEMA everywhere.

I'm in the UK so it's C13/14 or C19/20's only for me, we don't do NEMA here. The 13/14's are a bit loose you're right - that said I'm pretty lucky, I tend to just use blade enclosures so even if one fell out if wouldn't really effect our machines. If you have the choice then go with whatever makes you sleep best at night.