But basically, liquid comes in more strains that can help you make certain styles better but is more expensive and usually requires a starter. Dry yeast is far cheaper but has fewer varieties available and you can almost always pitch directly or with just rehydrating in water.
Both make excellent beer, so it just depends on what kind of beer you're making, how much you want to spend, and how much time you have.
I am usin my first liquid tomorrow! Have a starter goin right now, just checked and lots of yeasties. My first brew I used an activator pack of Wyeast German Ale and this is White Labs liquid burntons ale