Burr grinders are preferred over blade grinders for a couple of reasons. The most important reason is that a burr grinder crushes the beans between two burrs (gear-looking disks). When you adjust the grind size, you're moving the two burrs closer together or further apart. In doing so, nothing larger than the space between the two disks gets through, which results in a relatively consistent grind size.
With a blade grinder, the blades whirl around and smack the beans and shatter them. To control the grind size, you simply grind longer to get a finer grind. But different coffee beans are different sizes. For example, Maragogipe beans, also known as elephant beans, are huge and would reguire longer grinding times to reach the desired size.
Another issue is the whirling blades can actually heat up the ground coffee and start chemical reactions that affect flavor in an unpredicatable way.
There are numerous burr grinders out there with several options. We use a Baratza Vario-W that allows us to pre-determine the weight of the ground coffee we want and to very accurately determine the grind size. But you can also get an inexpensive Cuisenart that will do just fine.
So remember; keep the blade grinder for your spices and use a burr grinder for your coffee. You'll get a more accurate and consistent grind size, which in turn will result in a more consistent, enjoyable cup of coffee!