The LH351D 70CRI is slightly more efficient than the XP-L2 in most cases, unless you are going crazy with direct drive, but then, efficiency takes a nose dive with the XP-L2.
Seems like the lower thermal resistance of the LH351D really helps in at higher lumen levels. At 6A, the LH351D produces 180 lumens more than the XP-L2, which is really helpful for maximum thermal dissapation.