Since I know the load will only be in one direction, I've found the Klemheist knots to be more efficient than prusiks for connecting my tarp to a CRL.
With the prusik, half of the wraps are bearing the majority of the load so you are increasing the amount of cord necessary to get a "lock". On the other hand, the Klemheist's uni-directional application maximizes the holding power of all the wraps which is more efficient. Additionally, the Klemheist pulls more in-line with the CRL since it pulls from the end while the prusik creates a kink in the CRL since it is pulling from the center of the knot.
Either one will work fine, of course...this is just my preference. HYOH