Russ,
The biggest difference in most of the screwdriver type
antennas is mechanical, how it's put together, how well it
will stand up under use (and price!). Electrically, most
are basically the same except in power handling ability.
It's been a few years since I did any investigating of
screwdriver antennas so what I know about them isn't
exactly current, but I seriously doubt if there have been
any great 'break-throughs' in design. In general (meaning
that there are always exceptions), pick one that you 'like'
the looks of, can afford, and won't tip your vehicle over.
All the rest is just advertising 'hype'...
'Doc