Besides consistency, the paste has the most concentration of cleaning power. The paste also has an added ingredient for longer lasting protection. This protection will keep your shine the longest.
The liquid is not as concentrated as the paste, but will give you comparable polishing results on most surfaces. The protection will not last as long as the paste, but it will last longer than most other liquids on the market today. The liquid polishes cover more area for less money, so they are the best choice for large areas like boats or RVs, and areas that you must maintain on a more regular basis, like brass rails or sinks. Many people find the liquids easier to work with because of the spread-ablility.