Like Back Alley says, the baselength and viewfinder are the biggest advantages. The R2a viewfinder is real good though; even with spectacles, the 35 frame-lines are visible. On the ZI, there's just more space around the frame.
On the minor side, the strap lugs are also better placed on the ZI. The R2a accessory grip provides a new lug in a better position but blocks the tripod socket.
On the R2a, the tripod socket is placed centered under the lens. On the ZI, the socket is curiously on the right-hand side a la the Leica M. However, the ZI grip accessory puts it back where it belongs.
Also, the meter display on the R2a is more visible than on the ZI. The ZI has it on the far left side in the viewfinder and the R2a has it on the bottom.
Having both cameras, I feel better about using the ZI with my 85/2 but wouldn't hesistate to use the Bessa with any of my 35's.