I guess, your line of logic was:
1. one RAID-1 volume 74 GB, more reliable, not necessarily faster
2. one RAID-5 volume 222 GB, fast, reliable too.
Since they are two volumes, there is a hard line between them. Now the question is how to partition and which partition goes to which mount point.
In a Moodle-Server there are two directories which will grow with the usage: $moodledata, $mysqlvar. Administrators have no control over them.
Then there is the $httpdlogdir, $syslogdir where the administrator can do regular cleaning.
Thirdly the $tmp which also shouldn't grow out of control.
$moodledata and $mysqlvar both require speed, and can grow indefinitely. So I would put one or both of them on the RAID-5 volume (222 GB) and everything else on the RAID-1 (74 GB).
Now comes how many partitions, where to mount them, what really are $moodledata, $mysqlvar, $httpdlogdir, ... For that Chris gave a quite a few pointers in a previous posting.
Hope it helps.