The core for both PG and Oracle are on about the same basis in terms of stability. There are some packages that are in slightly worse shape for PG. On the other hand there's a community-developed package called "edit this page", a kind of "content-management lite", that at the moment only support PG ("ETP V2" will support both). So there's give and take.
dotLRN is still under active development, and the port to PG trails Oracle development. If you track it you'll see that the Oracle version is still a moving target, though the PG version does indeed have to move further before completion.
Sloan is planning to use it this fall under Oracle and the PG version should be in pretty good shape by then. If you try to use it for what you're describing this fall you'll still be in "early achiever" status but things should be hanging together quite nicely.
