Radiating efficiency is the most important characteristic to consider. For
short mobile antennas radiation patterns are all the same, the pattern in
all planes much depending on where the antenna is mounted on the vehicle,
and not on any peculiaritiies of the antenna itself. A roof-mounted antenna
can be very different from one mounted on the bumper bar although you may
not notice much difference in practice.

The most important dimension is antenna height. All other things remaining
unchanged, the higher the antenna above its base, the greater the radiating
efficiency. Low antennas are quite sensitive to height.

Loading coil position is not critical. Anywhere between 1/2 and 2/3 of
antenna height will provide high efficiency. Avoid heights greater than
3/4.

The physically larger the loading coil the lower the coil loss, the higher
its Q and the greater the radiating efficiency. Double all coil dimensions
and wire diameter, reduce the number of turns, and coil Q is doubled at the
same frequency.

'Screwdriver' type antennas are best when the unused short-circuited portion
of the loading coil is enclosed in a metal tube, and the used portion is at
a height about 1/3rd to 2/3rds of the antenna. Again, highest efficiency
occurs with the largest diameter coils with the largest wire diameter.

If one antenna looks something similar to another then radio performances
will be indistinguishable from each other. So choose on the basis of which
one gives the impression of being more mechanically reliable.
----
Reg