Mabul Island has several resorts specifically catering to divers; it also has less expensive hostels for backpackers/hitchhikers who wish the less expensive lifestyle. There are trips/tours daily over to Sipadan supporting that diver community as well.
Mabul is commended not only for its closeness to Sipadan diving, but for it's own diving as well. Although not as clear as one would want (for many reasons, mostly man-made issues) there is plenty of undersea life readily available.
Si Amil island has a pier and some sunken WWII jetsam. This island is not permanently inhabited, and is only reachable by boat. There appears to be pretty decent diving there as well, although it's not hailed by many as a "spot to go to".
All in all, it's kind of hard to tell if Mabul is recommended because they "want the tourists to go there as an alternative to Sipadan" or if it's truly great diving. Conversely, is the lack of recommendations of Si Amil diving because there's nothing to see, or no one living there needing the tourism money.
If you have the time and wherewithal, I might recommend both! Then blog about it. :)