Koscheck is a much better wrestler than Matt Hughes. Just like in other styles there is a big difference in "world class wrestlers". That term is overused alot I think. The announcers say "world class kickboxer" or "world class wrestler"..etc..
With that being said I"m not sure Koscheck is well rounded enough yet to take on Hughes. I"m pretty neutral on Koscheck(he acted like a punk on TUF,but is also a hard worker/trainer,so I"m split a bit),but the guy has a chance to go for a title in a few years if he picks up some more skills.
Thats a big difference between Diego and Koscheck. Diego isnt in the same class as Koscheck in wrestling,but Diego is the much better MMA fighter. Give Koscheck a couple more years and some more fights and he very possibly could be a top 3 fighter in his weight class.