The Alpine Class D is a bit more efficient. Its four-channel and puts out its full rated power into 4-ohms so there will be no change in efficiency, damping, distortion, etc as applied in this case. Other topologies will marginally lose efficiency, lose damping and increase distortion at a lower 2-ohm impedance. The Alpine amplifiers ship with a birth certificate that routinely demonstrate that they deliver a bit more power over their rating. The Arc test report shows that it exceeds its rating too. Neither of these amplifiers will be delivering the same power from the typical voltage supply available in a boat. It will be less. If we are going to hear a difference in power it is going to have to exceed a 33 percent increase to constitute a 1 dB difference. We are talking here about a smallish 6.5-inch HLCD that is dominated by the horn with little midbass and balance so I doubt we'll hear a difference in tonal qualities between these two amplifiers. It could be different with another tower speaker. The minor efficiency difference may not show up in the context of an entire system depending on all the other components and potential issues that also affect efficiency.
If I could find one tiny difference it would be that if you seriously abuse both amplifiers I might put my money on the thermal capacity of the Arc Audio amplifier. The Alpine might self-protect sooner. Or, you can flip a coin.