I agree with the fact the Wl-HDD is too slow for concurent connections.
In the case of our training room, the 6 concurent connection would be if samba couldn't fork more than 6 time (which is not the case) So in a way, it is possible to have 30 concurent connections, as there will not be any firmware lock. But each one will be so slow it would need to wait for one to finish to start one other.
On the other side, as a trainer, you get a DHCP, DNS, WINS box and some FTP, HTTP and SSH services which is good to demo, and lighter to take with you.
In a few words, if it use is for casual file copying, and mostly server services on the go, the the WL HDD is the thing to get because of it's form factor.
If you need file sharing for a production environment with progtram installation, go for a real laptop or computer.
My guess is that you only need a few word documents to save on the box. The WLHDD will happily do the job for this files, even for 50 students.
As for the user account, I think it is not the problem for you as you just need concurent connections. In any case, you can use latest Oleg's firmware to set up as many user account for file sharing as you want (well at least a few hundreds
)