The type of software normally used in cars will not be much use in an aeroplane as it is invariably "road" based with little other information.
Probably the best mapping software is Memory Map; it utilises UK 1:250000 charts 1:500000 aviation charts (at a price) and 1:50,000 charts if you want more detail.
Its usefull on the ground and in the air in that it shows you where you are, but not much else.
PDAs even when fully bright can appear quite dim in an aeroplane or car on a sunny day!
The first thing to ask yourself is What do you actually want it to Do? Then look for something that does what you want. The PDA is a good solution if you don't really know, and want flexibility to experiment. They are not very robust and need a power supply to keep them going; the connector is usually the weak point of the system! Garmin units can be much more robust, tailored to various needs such as aviation, but the mapping may not have the same detail.
The mapping software will probably cost more than the GPS Unit!