The point of maximal mechanical tension when doing cable curls is when your forearm is at a 90-degree angle to the cable. This angle occurs closer to the mid to shortened range when facing the cable.

However, when doing the face-away curls, it occurs more in the lengthened to mid-range. To build full-range strength, do both in drop session fashion:

One-Arm Low-Cable Biceps Curl Drop Set

You'll do both of the exercises unilaterally with a cable handle attached at ankle level.

1. Perform single-arm biceps curls using a weight you can do for 8-15 reps.
2. Once you've done both arms, go back to the arm you started with and do face-away curls while using a weight you can do for no more than 8-15 reps.