Also worth noting that should you be looking for servers, 1u will be cheaper to host at the hosting datacenters which charge by the U
This is not 100% true, but mostly, yes. I haven’t had anything than full racks in a few years, but at least in Germany I’ve seen different offers, sometimes it differed a little, sometimes it was really 2U = 2x 1U.
I always thought the short 1U servers were for shallower racks, like the network boxes you often see on walls.
The normal Dell rails for their short 1U servers are still full length (although perhaps you can get special shorter rails if you ask), so I don’t think they’d fit in short racks. Also, I believe that short telco racks are often only front-mount (with no back set of posts), which doesn’t work for server rails. My best guess about why Dell appears to like shorter 1U servers is ‘less metal for the case’, which presumably translates to cost savings.
(I’m the author of the linked to entry.)