I have used a few of the HK 3,0 blocks for i7 builds so far (CU versions), and am pretty impressed with the results. Mounting system is also top-notch, just be sure to buy the matching backplate for lga 1366 (it's not included )
I don't have any EK Supremes with 1366 mounting though so I couldn't do any comparisons between them. I really like how you can easily optimize the HK 3,0 for single-die and multi-die CPUs by inserting or removing a separation plate into the block