XR15: Pros: You're going to be able to get evenly-spaced horizontal coverage. Cons: This requires six power bricks and outlets.
XR30: Pros: Only three lights and three outlets. Cons: Power bricks are larger, not quite the same horizontal spread, higher upfront cost as you'll need 3 lights to start.
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Honestly, you're not going to be happy with the coverage of a pair of XR30s. You're going to find noticeable gaps/dark spots. I have a pair of XR30s on my 71" 750 XXL and if I were to remove the XR15 in the center my corals would start plotting to kill me.
Here's how the mixed XR30/XR15 lighting looks on my tank with the RMS mounts (standard 8" height above the water). Note that with the XR30 the RMS mounts do "sag" a bit more than the XR15 (probably 0.5").