The only GPS I've used a lot is the Garmin GPS III pilot which has a fixed land basemap (ie: cannot be updated) and a Jeppesen aviation database. The latter can be updated by one off updates valid for one month or by taking out a 12 month subscription giving you monthly updates.
For myself I purchase updates about once every six months. Because I don't use the GPS for primary navigation this is quite sufficient for my needs.
The GPS III has the great advantage for me that it will sit in a pocket or my flight bag taking up very little space and if I get stuck I can switch it on and it's up and running in a couple of minutes from a cold start.
All of the portable Garmin aviation units use the same databases and I think the same applies for most of the others .
The Garmin 196 is very good with a big clear display but still protable, the colour 296 is very sexy with topgraphical info also (but at a price). In the past I've use a SkyMap which was excellant and I know others swear by the Magellan range.
Best policy is to go to a dealer where they're all on demo and give them a try. A good tip is to see if you can drive a unit without using the instructions.
In short a portable unit is invaluable as a backup aid.