When the turbo comes on boost the shaft is 'thrusted' forwards, the load is put onto the thrust bearing (normally a thrust washer between the 2) a 360 bearing offers more support as the the load is spread over a bigger area, think of a 270 bearing as a pizza with a slice missing, 90 degrees of the support area is missing of there.
Some modern turbos have 360 bearings with big oil cut outs, this can cause premature wear.