The idea behind prioritized bit rate is to support for each bearer, including low priority non-GBR bearers, a minimum bit rate in order to avoid a potential starvation. Each bearer should at least get enough resources in order to achieve the prioritized bit rate (PRB).
MBR i.e. Maximum Bit Rate is to restrict the traffic flow in a particular bearer. If the traffic exceeds this then it will be dropped. ( MBR does not apply to non-GBR bearers, only GBR bearers.)
MBR is only for GBR bearer where is PBR is for GBR and non-GBR both so they can not be equated theoretically.