My 2-cents:

1) If you have a low band apply valve body, the rear billet servo is a good idea as it can crack.
Even without the billet rear servo, shimming the stock servo to be solid is helpful; just be careful putting it in reverse (go to idle).
2) The front servo from the pre-'71 trans is a better race part. So, putting an aftermarket pre-'71 style is a good upgrade. If you have a pre-'71, dual spring, non-load servo, it is fine.