I have first hand experience with any of the systems, but here's another data point for a competing system from a different list:

At Oshkosh year before last I purchased a TPAS collision detection product
from Aircrafters.  The unit is manufactured by SureCheck Engineering.  It
always gave lots of false readings.  (You don't want to learn to ignore one
of these type devices.)  I sent it back to SureCheck a couple of times for
adjustment or repair but it just couldn't' seem to handle the DME in the
KNS80.  To make a long story short, I kind of gave up and was going to put
the unit on EBay but thought I'd drop them a line first to see if they had
any new solutions.

They offered to take the old unit (a year and a half old by now) in on trade
against the new model of my choice.  They were very generous on trade in
value.  Their new unit (I got the DR model - does not indicate direction)
and it does not suffer from the problems of the old.  I get an occasional
false reading but when it says less than a 1/2 mile, I now pay quick
attention.

They have been very reasonable to work with throughout the process.  They
stand behind their products and have treated me more than fairly.