There is actually an enormous difference in the types of proppants used in frac jobs.  There are a number of properties that are of importance when considering proppant: grain size distribution, roundness, and compressive strength to name a few.  I'll try to summarize the enormous scope of proppant choice in just a few sentences.
 
The purpose of proppant is to provide a high conductivity pathway for hydrocarbons to flow from the reservoir to the well.  Proppant does this by preventing the fracture from closing due to the stress of the surrounding rock.  The deeper a fracture, the more pressure there is in the surrounding rock trying to close the fracture.  If one where to use a low grade proppant such as brady or white sand at depths of 10,000+ ft as in the Bakken, the confining stress of the formation would crush the sand and turn it to powder and it would no longer have conductive properties to allow fluid flow.  By pumping a higher strength proppant in a frac job (sand being the lowest strength - generally, resin coated sand being the intermediate strength, and ceramic and sintered bauxite proppants being the highest strength) a propped fracture will allow fluid flow even at extreme closure pressures.
 
It should be noted, however, just because a high strength proppant (ceramic or sintered bauxite) has a higher compressive strength doesn't mean it is a better proppant in all cases.  If one were to pump a high strength proppant at shallow depths, you would actually make a poorer well than if you pumped sand.  Not only do you pay more for the high strength proppant than you do for sand, but at low confining pressures, sand is actually more conductive!  Believe it or not, there are many Ph.Ds who spend their entire careers studying proppants and how to choose the best proppant.
 
To put it simply, however, the deeper the producing interval, the greater strenght proppant is needed - which corresponds to a higher cost.  As you might imagine, a company can spend hundreds of thousands of dollars on proppant for a fract job.  Keep in mind that the company wants to maximize their investment and tries to choose proppant by optimizing the economic benefit realized by increased production versus the cost of a proppant.  It should be noted, however, that with oil at $100/bbl (or, I guess $81 today), proppant can be in very short supply.  Companies can't afford to let their investment (the well) sit idle for months while they wait for proppant to become available, so often they will choose the best suited proppant from what is available.  This may be a higher cost proppant or a lower cost lower strength proppant.