[OSM-newbies] Garmin eTrex 20/30 - is altimeter important for recording GPS data for mapping?

[Sebastian Arcus]

I was looking at the eTrex series from Garmin to use for mapping on OSM, 
as they seem to be robust, compact and have specially long battery life 
(25 hours on 2 AA batteries). The main difference between 20 and 30 
seems to be that 30 has altimeter and compass, while 20 doesn't have 
them. Is this important when recording GPS tracks to use for mapping on 
OSM? Doesn't compass and altitude data come from GPS satellites anyway? 
Or an internal altimeter and compass are more accurate?

If anybody here has either of the above GPS units and uses them for 
mapping, it would be useful to hear a first hand opinion.

Thanks,

Sebastian