I recently discussed this with someone else.
apt-get: works much faster when you want to install something, but the package list needs to be updated periodically.
yum: accesses the mirrors everytime you need to install something or update so it works much slower, but always has the latest lists.
The difference is almost negligable since the best repositories support both. It really depends on how you use it. If you do a lot of installing, I'd use apt. However, if you only run the update periodically, yum might be a better bet.