...Which is the better antenna for use outdoors the "J" pole or the ground plane on 2 meters/70 CM...
Both antennas are essentially a half wave vertical radiator, so from a theoretical standpoint
the performance will be nearly equal.
Both antennas are prone to common mode currents, which means that they may work
considerably worse than expected. (Such currents on the feedline and/or mast can
raise the angle of radiation and cancel some or all of your signal at the horizon.) So
the biggest difference will depend on the exact construction of the two antennas and
how well they are decoupled from the feedline and/or mast. My guess is that it probably
is easier to provide sufficient decoupling for a ground plane than for a J-pole, at least
for the common designs found on the internet. But sometimes it isn't a trivial task,
and may require a second set of radials 1/4 wave below the first.
Beyond that it will depend on the construction materials: copper will turn green, but
soldered connections won't corrode as badly as with clamped aluminum joints. (Copper
to aluminum joints are particularly bad.) Insulators need to be able to withstand
exposure to the sun, and in some cases that may limit the usable lifespan. The mechanical
properties of the materials are also important: #14 wire used for radials won't last as
long in the wind as 3/8" aluminum rod. (It also depends on how many large birds such
as owls or hawks are likely to perch on it.) The coax connection should be protected
from the rain and snow as well as being waterproofed.
In the end the details of the specific implementation will make much more difference
in performance (especially over time) than the choice between the two designs.