The values returned by both the OID's are same most of the time. Sometimes, they might slightly differ due to rounding errors.
The OID 1.3.6.1.4.1.161.19.3.1.12.1.29 (frUtlLowMumimoDownlinkSectorUtilization) is supported on PMP 450m radios only. If you are polling a PMP 450m AP, you may want to use this OID.
If you are using polling a PMP 450/450i AP you may want to use 1.3.6.1.4.1.161.19.3.1.12.1.1 (frUtlLowTotalDownlinkUtilization)