In electrical terms it doesn't really matter, assuming the locations
offer a ground-plane that is more or less identical. But the fact
your airplane did not come with an antenna array already installed
begs the question as to type, age, method of construction and so
forth.

For maintenance however, antennae mounted to the top of the structure
usually offer some benefits. But as always, this is a general rule;
you will run into exceptions... without which this Group could not
survive! :-)

Just be careful not to measure Field Strength readings for a system
using the lower side of the hull, since the proximity of the earth
will skew your readings. Your 'Free Space' radiation pattern should
be identical, top or bottom. But here again, that 'Free Space' only
appears in text books. Ain't nothing 'free' nowadays :-)

-R.S.Hoover