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So, [controlled] liquid water is better but [controlled] solid water is more abundant?

I wouldn't say liquid is "better". The neutrinos don't care from a cross section standpoint.

Uniformity of the light field is going to be different, but that is not my sub-domain.

A liquid you control (and can densely instrument) is going to be a much easier to characterize detector than large volumes of natural material

Wouldn't the higher density of liquid water be an advantage?

It's quite a minor difference