Who is at Monsters of Rock? Are there bands that you like very much and you propably aren't able to see in the next years because they don't tour? If so you should go for Monsters of Rock then since BJ are touring next year.

It's a difficult decision I know but if there are bands you really like at this festival you should go to the festival. If there aren't bands where you are really interested in, sell your ticket and go to BJ.