The XBox One and the PS4 are brand new consoles, so for the moment, there aren't going to be as many games available for them. OTOH, any new game releases in the future will be written for these consoles, so over time there will be a huge selection of titles to choose from. These consoles are not backward compatible - games written for the current generation consoles (the XBox 360 and the PS3) cannot be played on the new (soon to be current generation) consoles. Sony has discussed making some older games available for the PS4, but that will be handled by hosting them on their own servers and streaming them over their network, and the number of older titles will probably be quite limited.
The decision comes down to which games you are most interested in playing. If you're going to play older games and the majority of games that are on the market right now, you're best off sticking with one of the current generation consoles (XBox 360 or PS3). If you're mainly going to play games that will be coming out over the next few years, you need to be looking at the XBox One or PS4, and waiting for the game releases.