I think that the choice between these two is dependant on what you see as your priorities for future use of flash in your photography. Both flashes have red LED AF assist lights and can do slow speed synch which will allow you to balance indoor background and subject exposures so your subject doesn't look like he's in a dark room.
If you only want a more powerful and versatile flash for general picture taking, and don't see the future possibility of using multiple external flashes, then the Sigma is probably the better bet. It's about half again more powerful than the Af360 FGZ and the ability of the head to swivel will help with bouncing the flash for portrait oriented shots. The extra power would be useful for bouncing in rooms with higher cielings and at greater camera to subject distances, or when using diffusers or bouncers at greater distances. It's also less expensive.
One of the potential negatives is that Sigma apparently reverse engineers Pentax's P-TTL technology as opposed to licensing it from Pentax (as indicated by their admitted incompatibility with K series bodies and the free rechip update offered). They obviously didn't get it 100% right the first time, and potential incompatibilty with future bodies might crop up. As long as they're willing to correct problems that arise, then this problem isn't really serious.
The AF360 FGZ has a number of more advanced features. The major negatives are lack of swivel head, the fact that the head locks in the horizontal position and you have to push a button to raise it for bounce (the latter is just an annoyance), and less power (but it's still about 8 times more powerful than the onboard flash). On the plus side are:
high speed synch which allows you to balance the exposure between subject and background in outdoor fill flash situations,
a built-in optical slave trigger which allows you to use it as a manual slave to another non-P-TTL flash (it fires on the first flash, so it won't work correctly with the onboard flash as that is always P-TTL)
wireless TTL compatibility when used with another wireless TTL compatible external flash, which allows fully metered multiple flash use.
trailing curtain synch (which fires the flash just before the shutter closes as opposed to the normal leading curtain synch which fires as the shutter opens - this can give you a subject moving across the frame with a slow shutter speed a trailing motion blur with the subject frozen at the end of the blur).
Auto Thyristor mode which uses a sensor in the flash to meter the exposure and eliminates the preflash of P-TTL if you run into a subject who always gets caught blinking when using P-TTL. You do have to match ISO and aperture manually, but the info on the flash will tell you what to do.
Practically, the High speed synch and Auto Thyristor mode are probably the most important features for the more casual flash user, but if you really get into using flashes as a totally controllable light source the wireless TTL capability might be a real cosideration.
Only you can decide which features (or omission of features) might be important to you.
Sorry for the long post -- I figured that some of the features needed some explanation.