For applications where prospective students upload transcripts, is there a preference between scanned official vs unofficial transcripts?
If an "unofficial transcript" is requested, generally they just want something that contains the relevant information: courses taken, grades, any disciplinary sanctions, etc.

It might be most convenient to scan an official transcript, because then it will be in the most familiar format, but as far as I know it is also fine to, for instance, retrieve your own records from your university's system and print the relevant pages to PDF.

On the other hand, transcripts are usually printed on special paper that causes words like "COPIED" or "VOID" to appear on any copies or scans that are made.  Of course if the department only wants an unofficial transcript, they should not care that it is a copy, but sometimes it's hard to read a transcript if it says "COPIED" all over it.  So whatever you send, make sure it's legible.

Of course, as you say, an official transcript (sealed, by mail) will usually be demanded if you are accepted and/or decide to attend, and woe be unto you if it does not match the unofficial transcript you provided with the application.

It can't hurt to contact the department and ask what they would prefer.