Just like deciding where to live, you have to choose between the big city and the small town. Big web sites, like big cities, offer variety, but they can be overcrowded and easy to get lost in. For potential buyers, who generally like to browse sites that are easy to navigate in, large sites can be overwhelming and frustrating. Another consideration when choosing a cyber-gallery is theme. If the body of your work falls within an obvious theme, such as wildlife, then it could be beneficial to associate yourself with other wildlife artists.
Commissions are another factor to consider.
They are held for gallery sales. Just like any gallery, they will charge a commission for work that they sell. Some sites will ask you for exclusive rights on your work, so that you do not offer the same piece on a competitor's site. If this is a concern for you, you should offer only prints and price consistently. Being part of a cyber art gallery AND having your own personal art web site, will likely produce more results than one or the other.