Things to consider when choosing a watercolor paper
I think there are there are at least three or four things to consider when choosing a watercolor paper.
the weight: 300 grams per square metre or 140 lb is usually considered the minimum for watercolor painting. Below this you may have problems with buckling this can depend on your style of painting with wet in wet painting requiring heavier paper. You can try to get around this buckling problem by stretching the paper before painting.
material: 100% cotton is usually considered best for watercolor painting. The cotton fibres are longer than wood pulp fibres so cotton paper is less prone to buckling. However cotton paper tends to be more expensive. Some cheaper alternatives are 100% cellulose paper, however cellulose paper is usually terrible for glazing so again it depends on your style of painting. You can also get cotton and wood pulp blends and other alternatives such as bamboo fibre paper.
surface: hot pressed paper is smooth it is good for fine details and a more illustrative style. It is not so good for for more painterly styles. Cold pressed or rough paper is better for looser impressionist styles of painting.
sizing: (maybe less of a consideration than the above three but still worth being aware of). Paper made especially for watercolor painting has sizing added either on the surface or as part of the paper manufacturing process. The sizing slows the process of the water passing into and through the paper.
My favourite brand is still Arches paper although it is not cheap.