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Well since Cisco's bandwidth statement assumes symmetrical bandwidth, you can either chose which ever bandwidth is most important to you for monitoring against 100% usage, or you can chose the higher value (which helps avoid usage appearing to exceed 100%).
However, bandwidth statement might be used by other protocols. For such, you would set as they might need. For example (generally by default), Cisco's OSPF uses the bandwidth statement for setting it's egress OSPF cost. So, for such, you would want to set the bandwidth statement to match your egress (up) bandwidth.
