This is a tough decision and ultimately one that you will need to decide on your own.
I would personally have a hard time trading two machines for one but condition is everything, and if the two machines you have are worth about the same as a decent AFM then I would consider it.
Keep in mind though that AFM is the rumored CGC next title which could have a negative effect on the current value of the Bally machine, just as MMR had on its original counterpart.