The reader's digest version of why we chose the 430:
It fits, is not that much more expensive, works well with the MX20, has a
superior user interface, and the display is very handy when used for
setup/lookup tasks. The map is redundant, and includes incremental info not
found on the MX20.

Unabridged version:
The Garmin NAV info is sent to the MX20, where it is combined with terrain,
weather and chart databases contained in the MX20. It's true that you can
put in a cheaper (and smaller) IFR GPS to interface with the MX20, but we
chose to go with the 430 for the following reasons:

- Combined with a SL30 Nav/Com, XPNDR, audio controller/Marker Beacon, the
430 and MX20 all fit in a single stack in our panel
- Subjective preference for 430 user interface over King KLNs, UPSAT MXs and
others
- 430 map page has better ID for roads, bodies of water etc. than MX20
- $$$ difference not all that great between 430 and others, if you consider
installation and certification costs

90+% of the time I use the combination of MX20 moving map+terrain+NAV
overlay with the 430 CDI page, yielding awesome SA for both VFR and IFR.
Our Sandel EHSI also displays the GPS waypoint, flight plan, groundspeed as
well as standard HSI info. Since flying behind the MX20/430 combo, I
haven't unfolded a sectional, terminal or enroute chart while in the plane.
Except to ID roads and lakes (on long boring flights), the 430 map page
doesn't get used all that much. The display on the 430 works well for
looking up NRST data for position fixes, waypoint info, setting up
approaches, and other miscellaneous stuff. We don't have the weather input
installed, so can't help you there. I'm not sure if the TIS info from a
Garmin Mode S XPNDR (GTX330?) can be displayed on the MX20; that or perhaps
a TCAD input would be really nice.

Here's a link to the MX20 documentation:
http://www.garminat.com/mx_docs.shtml
(garminat.com is the website for the APOLLO products)




If so, then it seems that the garmin 430 display space is a waste of
valuable panel
real-estate. If not, how are the two different displays used
differently? Seems like it adds to workload that way to have to monitor
2 screens.