The SR UxM Scarlet Witch: Controls Probability read:

While Scarlet Witch is active, you may reroll any or all of your dice one additional time during the roll and reroll step.

Both of these cards have advantages over such recently produced cards such as Lab Test, which read:

Continuous: You may send this die to your Used Pile to reroll one of the character dice in your Reserve Pool.

Thanks to some recent clarifications on the rules forum, we now have a better understanding of the differences between dice in a “rolled” state and dice in an “unrolled” state.  Rolled dice in the reserve pool, or the field zone are either character dice or energy dice (if they’re from character cards) depending on whatever face they are showing (the same is true for action dice – either action die or energy die depending on the rolled die’s face).  Unrolled dice in the Used Pile, Prep Area and bag are considered character dice or action dice depending on the card they’re associated with.  Because of this new understanding of the status of “rolled” dice in the reserve pool and field zone, it was determined that a card like Lab Zone wouldn’t allow you to reroll a character die on an energy face, because it wouldn’t be considered a character die, but rather an energy die.

Both of the Scarlet Witches avoid this problem as they both allow you to reroll your “dice”.  However, there still are some pros and cons for this new card in comparison to it’s original UxM counterpart…

Secret Wars vs. UXM – Scarlet Witch Pros & Cons

Cons:

UXM SR Scarlet Witch could reroll your dice out of the field zone providing the opportunity for all kinds of hijinks such as refielding characters or fishing for additional energy for rare Lantern Ring damage.

UXM SR Scarlet Witch also gave you an additional crack at rolling and potentially fielding a stubborn character die that you’d like to be able to field but insists on rolling energy…

Pros:

Both Cards increase the probability of you rolling what you need, which in a game of probability is powerful.

However, the common Secret Wars Scarlet Witch does have some advantages in comparison to its UXM counterpart.

1.  Because it’s ability triggers in the main step you can use her ability the turn you field her, unlike her UXM counterpart, who must become active and then wait for the next roll/reroll step.  In a game of tempo, this additional speed might be the difference in the game.
2. The common Secret Wars Scarlet Witch works great with ENERGIZE characters, because it allows you to get an energize trigger at the end of the roll and reroll step and then potentially get another one at the end of the main step when you trigger her ability.  Potentially doubling that energize ability.
3. This Secret Wars Scarlet Witch also harmonizes well with Action Dice, because actions can be used in the attack step, (it could be particularly effective with the new EPIC BASIC ACTIONS that you really need to land) – however because of it’s wording it might not be much help in getting that character that keeps rolling energy into the field, because her ability triggers “at the end of the Main Step” – as written that reroll would be the last action you could take in your Main Step (“at the end of your main step”) and thus you wouldn’t be able to take another action after that to field those characters that might have been fortunate enough to turn for you after using her ability (you can’t field characters from the reserve pool after the main step).  There are Attack Steps shenanigans to be had with Action Dice such as Mutation and the like, but that’s adding a level of complexity to the situation and doesn’t really mitigate this card’s particular limitations in this regard
4. Bottom line – this card is particularly suited to keyword abilities like ENERGIZE or SPARK.

Those EPIC BASIC ACTIONS are expensive.  A double ENERGIZE trigger on Forge could help you get there…