Evo7
I think that the difference between Garmin GPS models, are:
"Outdoors" is designed mainly for walkers, climbers, etc, & shows towns, roads, footpaths etc.
"Aviation" are designed for aircraft use, & has a database, of airfields, controlled airspace, etc. You can then buy regular database updates from Garmin, for a fee, for your GPS.
The "Aviation" model is well worth the extra cost, as it gives you far more relevant information, eg nearest airfield, (if you are having a panic!) & whether you are about to infringe controlled airspace. Without the aviation database, you would need to do a lot of data imputting yourself, in order to get anything useful out of it.