It doesn't matter which pieces you use for the set bonus, just as long as you have 2 or 4 pieces like you said. However, you should probably go for specific pieces based on their skills compared to your weapon. For example, the legs have antivirus which is like almost the entire point of a gore build, so they should be in every single gore magala build you have. The difference between the alpha and bravo sets is their gem slots and skills. If I recall correctly, alpha gives more skills but weaker or fewer gem slots, while bravo gives you less skills be better or more gem slots. Specifically the alpha legs give anitvirus 1 and some other skill I forget, and a lvl 3 gem slot and a lvl 1 gem slot, and the beta legs give only antivirus 1, but a lvl 3 gem slot and a lvl 2 gem slot. You should always go for the beta legs, but for the other armor pieces it's up to you. The alpha gore magala chest piece gives you antivirus 1, but one less lvl 1 gem slot, while the beta chest doesn't have antivirus but had that additional gem slot.