the same bearings will last from a few minutes to many hours in different clutches... radial clutches a la fioroni/BBF give you the longest bearing life expectancy of them all (tried and trued by yours truly).
I now buy mixed shields (1 rubber for the outside, one metal for the inside) 5x10x4 ABEC 3's (read cheapo bearings, less than $1 a pop)... my last set on my BBF clutch still spins like new after 4L of fuel with an agressive setting.
It's all down to the clutch - not the bearings. Remember to leave a little play (0.2mm axially).
Paul