well if you want the FULL and extensive information you can check out this thread: http://kofk.de/viewtopic.php?f=12&t=27368
but basicaly, knuffels give you money every time they lvl up. Now, they cost 2500fps, but by the time they've reached lvl 100 the total profit you gain is about 7,5K from everytime they've leveled up.
Additionally, when you dig with them, they can give you either materials which you can sell, digging avie items, and fps (from 10fps to 1K in each dig). Everytime they lvl up they can make more digs each day.
It doesn't matter wich knuffel you use to dig or what element he is, the items you obtain (including digging avie items) depend on where you dig.
Like, if you dig in the dark plains you'll get darkness mats and darkness clothes. Though if you dig with a darkness knuffel you have more chances of getting successful digs in the darkness plains than say, a fire knuffel.
AND, in the future, you'll be able to FIGHT with your knuffels =D how cool is that? ^^
That's more or less all you have to know ^^ as I said if you want more in depth information check out the link I gave you =3 but I totally recommend you to get several knuffels ^