Well, this is really an opinion based answer, as neither systems have come out with any kind of specs for the systems. However, there are some rumors/basic things that we know so far.

I am a Sony fan, so logically I have been keeping up more with the PS4, and I do know some things about it. First off, it will not have backwards compatibility upon the system's launch. This is sorta good and sorta bad. It will keep some Sony fans from buying it because they might be sacrificing the ability to play PS3 games on it. Then again, this will reduce the price considerably. Also, it sounds as if the system will have some sort of restriction with new and used games, where some parts of the game are only available for people who buy the game new. However, it sounds as if Xbox might do this as well, but this is unconfirmed for both systems.

In terms of what will win, I will say PS4 for a few reasons. First off, it will come out first, as vowed by Sony a couple years ago, saying that if they do anything, it will be getting the PS4 out before Microsoft's next system. Secondly, I believe that it will have better specs. Sony owns the Blu-ray hardware, and although Xbox is rumored to be using Blu-ray as well with their next system, Sony has more experience working with Blu-ray, and will probably create some better graphical games upon the launch of the two systems. Third, Sony has always, and will continue to have a considerably greater number of exclusives, which is something that Microsoft has seemed to struggle with.

Then again, we dont have any solid information on either system yet, so its really up to opinion.