You can usually identify a hard cam eccentric by the oblong shape. Because of their velocity, hard cams are more difficult to shoot consistently, and require higher maintenance. Intermediate to expert hunters and range shooters fancy hard cam eccentrics. They combine a top side wheel with a bottom end cam. They offer the velocity of a speed wheel, but do not pull as easily.
Single cams pull hard like a traditional cam, yet are easier to tune and shoot.
I recommend purchasing a bow offering either speed wheel or single cam eccentrics. Despite what your might here, or assume because of how different they look on various bows, there is little variation in performance from one type of riser to the next. Again, a riser is the handle portion. This forces you to pull the string further back, which in turn holds the string on the arrow for a longer period during the release, resulting in more velocity. Some inexpensive models with straight risers are difficult to tune.