Both terms can be useful, especially for planning purposes, and when
used together.
If you have an engine with known GPH at specific rpm's (+/- a small
percentage) you can easily plan ahead for consumption and reserves for a
specific distance at various speeds or for days of operation (where
mileage isn't a consideration) before needing to "watch for a gas station".
Although MPG/MPH is great for planning, it doesn't take into
consideration that it's "through the water", so that even when connected
to a GPS, the number will vary up and down.
In the end, a lot depends on type of boat, type of operation, and route
to be traveled.
To be honest, I'd use both for comparison. Most ships use B/M
(barrel/mi) with a reserve of 2-3 days .... always calculate some degree
of reserve.

otn