I think that the (Reversible) method follows the books closest. Perhaps this is following the 2nd edition closer than first, but I think that that gets the point across.
I there that may be some cases where you will have multiple choices on the way the spell is cast, whether that means like casting a darkness/light spell on a target, on an area, or targeting a persons eyes (for a blindness effect); or perhaps something like the Prismatic Spray spell (I *think* this is 2nd edition spell aka its not found in 1E, but I could be wrong).
Prismatic Spray (7th Level MU)
Components: V, S
Casting Time: 7
Area of Effect: 70 x 15 ft. spray
Saving Throw: Special
When this spell is cast, the wizard causes seven shimmering, multicolored rays of light to flash from his hand in a triangular spray.
This spray is 70 feet long and spreads to 15 feet wide at the end. It includes all colors of the visible spectrum; each ray has a
different power and purpose. Any creature with fewer than 8 Hit Dice struck by a ray is blinded for 2d4 rounds, regardless of any
other effect. Any creature in the area of effect will be touched by one or more of the rays. To determine which ray strikes a creature,
roll 1d8 and consult the following table:
Prismatic Spray Results
1 = red
5 = blue
2 = orange
6 = indigo
3 = yellow
7 = violet
4 = green
8 = struck by two rays, roll again twice (ignoring any 8s)
Color of Ray, Order of Ray, Effect of Ray
Red, 1st, Inflicts 20 points of damage, save vs. spell for half.
Orange, 2nd, Inflicts 40 points of damage, save vs. spell for half.
Yellow, 3rd, Inflicts 80 points of damage, save vs. spell for half.
Green, 4th, Save vs. poison or die; survivors suffer 20 points of poison damage.
Blue, 5th, Save vs. petrification or be turned to stone.
Indigo, 6th, Save vs. wand or go insane.
Violet, 7th, Save vs. spell or be sent to another plane.
Glad that you are working on adding choice to 'at-cast-time'
ST the choosy