I think either works just fine.
That, but if you put a gun to my head, I'd lay it down like this:
Blackwater Park is a better choice if you wanna be able to enjoy more of their discography overall since Ghost Reveries' general style is more on par with their BWP-present output whereas BWP isn't that radically different from anything except for Heritage and some of Watershed.
However...
Ghost Reveries is a good choice if you're looking to get into their more current output (GR-present as well as whatever comes out in the future barring an unlikely return to their Orchid/Morningrise sound.)
It all comes down to whatever best suits their desires when you take the average of the two factors:
1. Do I prefer being able to get into more of their material or would I rather just get into the latter-era style and have less to enjoy in case I get a little stuck on that style thus making the heavier earlier stuff potentially less palatable.
2. Disregarding point #1, what style do I like more in general?