Carat is a term that refers to the weight of a diamond. Prior to the twentieth century, diamonds were measured using carob seeds, which were small and uniform and served as a perfect counter weight to the diamond. The word "carob” is the origin of the word "carat" that we use today.
The size of a diamond is proportional to its carat weight. When rough diamonds are cut and polished into finished diamonds, up 2/3 of the total carat weight may be lost. Since larger rough gems of high quality are found less frequently than smaller rough gems of high quality, a single two carat diamond will be more expensive than two one-carat diamonds of the same quality.
In the United States, the majority of diamonds used in jewelry and sold as loose diamonds are one carat or less in weight. The average engagement ring diamond sold in the U.S. is less than 1/2 carat in weight.
A diamond will increase in weight much faster than it increases in actual "face-up" diameter. For example, while an ideal cut one-carat diamond measures approximately 6.5mm in width, a diamond of twice its weight measures only 8.2mm wide—less than a 30% increase. The graphic below helps illustrate this point.
This question has no direct answer. It is a choice that depends on personal preference and budget. When looking at a diamond engagement ring, what is most visible is the size of the surface area on the top of the diamond. It is difficult to measure a diamond’s carat weight simply by looking at it. Although carat weight influences cost quite a bit, it is advisable to focus on diamond cut and diameter.