Andrew Ng gives a nice rule of thumb explanation in this videothis video starting 14:46, though the whole video is worth watching.
Key Points
- Use linear kernel when number of features is larger than number of observations.
- Use gaussian kernel when number of observations is larger than number of features.
- If number of observations is larger than 50,000 speed could be an issue when using gaussian kernel; hence, one might want to use linear kernel.