If you are looking for a GPS for geocaching, then you have 1 question to answer...
Are you going to do only urban caching?
If the answer is yes, then you can go with a droid or iphone or maybe even an ipad with the appropriate apps..
However, if you may be going into any environment where the rugged or weatherproof qualities of the equipment may be a concern, then you want a dedicated GPS made for such activity.
Many caches are hidden in trees, on hillsides, along rugged trails, in rocks, etc. If you are lugging about an ipad, you'll have problems because you may well need both hands to maneuver. Plus, all these non-rugged devices can be easily broken or damaged by water if you got caught in the rain, fell in a creek, or dropped them in a pond. You also have to consider that these devices rely on some form of wireless communication, be it cell or WiFi. There are many places out there where caches are located that these devices won't work as expected.