The SN21G5 is a standard G5 SFF case with the usual goodies, a PCIe 16x slot, one PCI, and enough USB ports to satisfy most sane people. It has 2 SATA ports, a single IDE chain, and a floppy port. It also sports 5.1 channel audio, Firewire, and the integrated graphics that come with the 6100. The only thing lacking is an eSATA port, but that is quite forgivable. If you are looking for a tidy 939 based system, keep this on your short list.
The SN36G5is more or less the same thing in LGA775 flavor. It is more a media center PC though, the main differentiator over the SN21 is that it comes with the front panel display that for some odd reason people seem to like on media centres. Functionally, it is the same as the SN21, it only differs in minor specs like 7.1 audio instead of 5.1, and GBe instead of 10/100. Again, a solid bet if you are looking at an Intel box.
Both releases look to carry the Shuttle banner for the current generation of chipsets. From my personal experience, they are solid, easy to build boxes that will serve you well. If you want a full review of the SN21, check out The Tech Report's take here. ยต