A servo vs. A+ servo vs. B servo...what is REALLY necessary?

I will be swapping valvebodies at some point and will have easy access to the stock B servo. But my motor is stock.. so do I really need to spend $140 on a billet A+ servo?

Can I put in a factory Ford A servo?

What vehicles came with the A servo (to aid in looking it up?)

and finally, will it really help? I know all the built AODs have them, but for a stock application, will I see any benefit in performance and/or longevity?

Thanks everyone!