Since 1.8.8 they are the same and MinecraftForge.EVENT_BUS can be used for Minecraft and Forge related events. But in 1.8 you need to register them both. FMLCommonHandler.instance().bus()
https://github.com/MC-U-Team/U-Team-Core/blob/1.8/src/main/java/info/u_team/u_team_core/util/registry/CommonRegistry.java#L23