I'm planning on going mobile, looking at the FT-100 or IC-706, but
I'd like to go with something designed as a well organized system. So
I could go with the FT-100 / ATAS 120, or the IC-706 / AH-4, or
maybe either one with one of the various screwdrivers.

Dan-

The AH-4 is claimed to work from 40 to 6 Meters using a long whip. It may work
mobile as advertised, but that isn't where its strength lies. In my opinion,
it makes a better field day tuner, working into a random length long wire plus
a good ground. I believe most of the other alternatives will outperform it
while mobile, especially on 40 Meters!

I'm surprised you haven't received other comments supporting either Yaesu or
Icom. I have never worked with the FT-100. Reports have been mixed, with some
people being fanatic about it and a few needing repairs out of the box.
Apparently its a great radio if you get a good one. On the other hand, I have
read reports that its companion ATAS 120 may work, but is not exceptional in
mobile performance.

Reports on the screwdriver antenna performance is generally good. A friend has
a very solid one mounted on the rear of his pickup truck. There may be several
different ones on the market, so look for one that is a little more
substantial.

Although not as convenient as the automatic systems, the single-band antennas
work fairly well mobile, at least as well as they can, since they are short
compared to a full quarter wave. Examples are the HamStick and the Hustler.
Also, the Hustler has an option-bracket used to mount several resonator coils
at once, so you can work more than one band without changing.

73, Fred, K4DII