I like a stream as it's less likely to blow back in your face than a fog. Fog however, does cover a much wider area (but a bit shorter) and is easier to hit with. Foam doens't blow back either but gives you less "bursts" (a consideration if you're gonna carry a very small can). All work, but you must figure out what will be best for your use. Always test spray your can with a short burst to make sure it works. Mace can should be held with a "beer can" grip, and you push the button with your thumb. This makes for more accurate spraying than using your finger.
All the best