CPA Secure Chosen plaintext scheme The example for IND-CPA secure schemes given is generally:

for a random r, 


  Enc_k(m) =(r|| E_k(r) XOR m) where E is a PRF


But does the role of r and k really matter--i.e. isn't this equally secure? 


  Enc_k(m) =(r|| E_r(k) XOR m) ?
No, it is not equally secure.  Consider an instantiation where $E$ is instantiated with AES, and where the message is always 128 bits long.  Can you find an attack on this particular version of the scheme?

This smells like homework (I'm guessing?), so to avoid ruining the problem and give you a chance to learn from it, I'm just going to leave it at that.  I think you'll find the process of working out the rest of the details to be instructive: for instance, it might help you understand IND-CPA security a bit better and how a scheme can fail to meet this security definition.