I've started looking for a GPS that allows selecting (switch?) a
database for roads, aviation, hiking, marine, etc.

Bill, My Lorrance 1000 will do that- it just takes a few keystrokes, and to
get at the really small roads- like dirt tracks- you might have to remove your
'aviation' memory card and slip in the terrestrial one you will have made from
the included CD ROM. Maybe a 1 minute operation.

I use mine both for aviation and in the big truck I drive to find addresses in
unfamiliar places and it switches back and forth easily- as well as giving you
values in km,miles,or nautical miles- again easily switchable.

Jim