Workstation (or Deskside , which is typically assigned to servers). Desktop is a generic, catch all, nondescriptive category that folks use to classify mainstream personal computers with. Go to Dell's, HP's, IBM's, etc. sites and see if they don't draw a distinction.
In all of the years I've been in the industry, I've never known anyone to use the term
Deskside. I do realize what you mean by the term, but I'm accustomed with Mid-Tower filling that definition (capable of both desk, floor, or a shelf system attached to the desk itself for physical location).
I even indicated that previously to try and clear things up.

Guess it didn't work.
BTW, my familiarity with the term
Desktop originally meant unit that laid flat (i.e. original IBM PC, and almost always had the monitor sitting on top of it), but later included the Mini-Tower (and the monitor set beside it).
Just different nomenclature I guess, but it can help clear things up.
Workstations have historically been much more closer to entry level servers that are assigned to one person. Usually because that person has workload that makes them not so favorable to timeshare on a shared server, but a server level of workload.
I've not argued against this.
At any rate, it wasn't that long ago that the same processor was use for consumer, workstation, or entry level server (i.e. 80486 systems were that way, as the split began with Pentium Pro as I see it with Intel parts, as the Pentium II Xeon was the introduction of the
Xeon badge).
The Mac Pro is more so an XServe in a vertically oriented box that can sit on the floor instead of a rack as opposed to a mini in a bigger case. Would be closer to a 2U XServe if one existed. The Mac Pro has much more historical design motivations from the history of Unix Workstations than from the original IBM PC. It actually is a Unix workstation with a candy coated UI.
From a component POV, yes, it does (same DP Xeons, same chipset). But take expansion into account (in general), they've split a bit (i.e. servers using on-board graphics), and fewer expansion slots in some cases, depending on the actual size of the rackmount enclosure (i.e. 2U vs. 5U). This also includes space for storage, as workstations tend to require larger storage pools for that system, rather than off-load it to a separate storage server (less expensive to include it in the system if doesn't need to be available to the entire network).
By going to pedestal cases (mid or full size towers), they can add additional slots to the board, more HDD locations, and use commodity large value PSU's (low profile PSU's are more expensive).
Ultimately, going to a different physical arrangement for a workstation produced lower costs, which won out over previous system types offered by SGI, Sun,... And with the advent of Linux or Open Solaris for example, Intel systems can run UNIX (hacking OS X not considered an option in a business environment).
Reliability is not going to be same if the generic case has a jacked up thermal design or the components jammed into it willy nilly screw up what the case was designed around.
I meant in terms of each component (board, graphics,...). As per the case, it takes the right selection, as with anything else (it's just another component to me, and has to be selected properly for the intended system).
Assuming proper cooling and other considerations (i.e. good build quality parts) are taken into account, it's not going to be much different.
If building out of Xeons, the price difference narrows, especially for DP systems (still a notable difference for a SP Xeon system vs. the '09 Quad MP).
But I also mentioned there's compromise for that lower cost in the form of
user = all support (hardware and OS, assuming it's OS X or Linux; a Retail copy of Windows does come with support for a period of time; rather short, but it's there).
But neither a DIY system, or Hackintosh one at that, are acceptable for a business. That's a fools errand IMO.
An enthusiast OTOH, could be another story (if they're willing, have the time to do so when needed, and have the skills).