About 80:20 by units sold, I would say; leaning slightly more toward the latter by volume of drives sold, by the fact that businesses buy the larger NASes that take more drives in them, because they're actually somewhat serious about RAID recovery.
But you've got to further subdivide the use-case: a file server for a business is still not necessarily doing "random writes." You can create and replace as many small files on a filesystem as you like, and in a modern filesystem, you'll only be doing large batched writes to the disks themselves. It's only when you're writing within existing files—i.e. writing to a database of some kind—that you see a high number of random-write IOPS hitting the disk.
Mind you, there are a lot of things that turn out to have databases in them. A Chrome profile has a couple of SQLite databases in it. An iTunes library contains a database. Etc. These are the things that—if you stuck them on your NAS, and then pointed the relevant program at them from your PC—would constitute the NAS performing a "Desktop" workload. And, obviously, if you run Postgres on your NAS—or on a system that mounts a NAS disk over iSCSI—then that disk is doing a "Server" workload.
But the average business's use of e.g. shared Excel worksheets, does not "heavy random writes" make. Document/productivity software is almost always of the "on save, do a streaming overwrite of the whole file on the remote end" variety—which, again, is not a random-write workload, since the ftruncate(2) at the beginning allows the filesystem to grab a new free extent for the overwrite to write into.
Most software that knows that it can be used in a shared-remote-disk setting is built to accommodate that shared-remote-disk setting by attempting to minimize random writes (since this kind of workload has always been slow on the kind of storage used in NASes and SANs, especially when your filesystem or software RAID does checksumming.) These days, it's only the software that absolutely can't help it—that needs random writes to be performant at all—that still does them. And it's pretty obvious, to anyone using such software, that they're not using "NAS friendly" software. Mostly because it's so dang slow!
