The MBX5 and MBX5 Prospec are the same basic car, the Prospec has strength upgrades (thicker shock towers, CNC chassis braces, CNC alloy rear hubs and a one piece engine mount).
The MBX5R uses the same chassis (engine and radio locations) but has upgraded, longer shocks and revised shock towers and front arms to allow more travel and has all the Prospec level aluminum parts. Otherwise, the R is still very similar the the original MBX5.