Unfortunately both are very common. Most of the OpenSource software I've found uses Junk, and most of the Proprietary software seems to use Spam. New users may be accustomed to either...
The only "solution" I've found is to use one or the other, and to do so very consistently. At home I use RoundCube as well, and I standardized on "Junk" long before. If I had chosen "Spam" I would simply change the default locations and button labels on RC so it matches. At work we use Exchange/Outlook, so "Spam" was the clear choice as everything was setup for it already (despite the fact that Outlook's buttons and menu options are for "Junk Mail").
It may be possible, depending on your server configuration, to have both folders linked to the same location. I've found this to cause confusion, more so than picking one or the other (users asked "What's the difference" despite both having the exact same content).
INBOX is part of the IMAP standard, the other three have generally been standardized on those names, but I've also seen "Sent Items" instead of "Sent", "Deleted" and "Deleted Items" instead of "Trash". "Drafts" is the only one I've never seen a variant of.