The most important distinction between blade and burr grinders is the result:
Coffee beans circulate across the spinning blades. No matter how long or short the grind, there is extreme heterogeneity in particle size--that is, you will find larger bean fragments with near-dust-size ones, with everything in between. If you use a pour-over or drip brewing method, you might even see whole beans floating to the top. Not surprisingly, you'll get very uneven extraction from these different particles, no matter what the brewing method.
If you grind long enough to decimate all the beans, you'll find a lot of fine coffee dust and sediment in your cup. Too short a grind, and you get a lot of beans and bean pieces that can't present enough extractable surface area to the hot water, leading to a thin, weak cup. Waste of coffee, to be sure.
Also, it's almost impossible to grind any two batches consistently, since grind time is dependent on bean quantity, type of roast, variety of bean--well, everything. So today you'll get a disappointing cup, and tomorrow you'll get another disappointing cup for some other reason. Therefore I recommend a Conical Burr grinder.
Beans move through once. When you set your burr grinder to a certain fineness, there will be very little heterogeneity in particle size. (N.B. Spend more on the grinder, get a more even grind.) This allows you to experiment with the grind as well as the quantity until it's perfect for you, then replicate that recipe perfectly every time.
Some tips on finding the right burr grinder for your needs:
- Prices vary from $50 to $1500 for non-commercial grinders. I'm happy with my $350 Rancilio, which I've had for 10 years without a hint of trouble.
- How you like your coffee matters. A more expensive grinder will do better fine grinds, as for Arabic-style (i.e. Turkish, Greek, Lebanese) coffee. If you don't drink that, don't spend more for it. Most burr grinders perform best in the automatic drip range.
- The under-$100 burr grinders still manage to throw a lot of fine coffee dust into the grind. Do your research before buying.
- Consistency in grind is critical to good espresso. If this is important to you, be prepared to spend as much on the grinder as you spent on your espresso machine. It's for a good cause. Why hobble your good machine with sub-par ingredients?