C.O.
We have a Garmin. My parents have a Magellan (land, air and sea).
You need to find out what you are using it for - driving only - or will it give walking directions as well (meaning hand held)
I know with Garmin we have to pay to have updates. Which is fine. I know there are some that are free.
Screen size and voice - all options - if you do Geo-Caching - will it help you there?
If it's just for driving directions - then just a matter of what you can afford to spend and what you like. If you need more than just driving - and will be using it in a boat - get a Magellan Air, Land and Sea. It's expensive. But worth it if that's what you are going to use it for.