The MacBook Pro is simply more powerful and has more features than the standard MacBook does. It all comes down to what you will use the computer for and what you find to be an advantage to owning one over the other.
While all Macs are great computers (as Apple doesn't make crap), you may prefer to get a Mac that is faster, or more durable, or a number of other things. In that case, you could look at the MacBook Pro. The screen quality of the MBP is much higher than that of the MacBook, producing vivid color and contrast, along with a higher level of brightness. The MBP has a larger hard drive, faster RAM speed, and a faster processor. It also has the multi-touch trackpad, which simply makes using your computer more intuitive and convenient... not to mention cool.
So if you want/need the extra power or nice features of the MBP, go for it. Otherwise, if you don't think you need them, save yourself the money and get the standard MacBook. If you end up going for a MBP, the 13" is probably the most popular right now, as it's a powerhouse in an easy, travel size enclosure. The larger the screen, the more power the computer can store within it, at the cost of it taking up more space.