Quick comparison of Grissenda builds
Basic (well rounded), High agility, and 2-H: The predictable pattern emerges. 2-H build does a lot more damage (50+ when Fury III kicks in, with stun chance). High agility has highest defense (38 baseline, 52 with Resilience III active).
Can tweak the builds by sight changes in equipment (Sonnet gives Def, other rings give elemental protection etc).
All three builds benefit from Resilience, Cleave and Fury. Obvious 2-H builds need 2-H Expert. Shield users benefit from Shield Expert (there's a rumor that Companions might be able to learn some advanced skills so that may impact which basic skills to use). Bash and Whirlwind might be the least useful as they have cool downs and Grissenda can knock away enemies at the worst possible times (knocking a fire demon into a fire, knocking a dragon away causing Duel to reset to zero etc).
What belt to give Grissenda depends on which guild a player joined (+2 AGL from Thieves Guild, +2 STR from Warriors Guild).
OUTSIDE OF PURE DPS AND DEF
A 2-H Build Grissenda with solid equipment (+2 STR Belt etc) is likely the most dangerous of the build options, making Grissenda a force on to herself. At least at end-of-game levels.
However, if a person is playing with a 2-H Warrior, having Grissenda be a light or 1-H weapon allows using the highest tier of each weapon type and allows the use of Shield s. With two 2-H weapon users, no place to equip a shield, making any shield found in the game, no matter how awesome, near useless to the player.
If a person is using a Rogue, Grissenda as a high-def build allows her to take more damage so that the Rogue can back-stab, or attack from afar more often.
If a person is using a Cleric, a 2-H Grissenda might provide so needed melee damage.
A balanced Grissenda can switch between 1-H and Light weapons with little penalty, allowing her access to both types as newer ones are found.
Equipping detection enhancing equipment on Grissenda can help find (and avoid) traps, however, realistically, unless the main character is lop-sided (all str, no other traits etc), Grissenda won't be the one finding traps and secrets.
There are a lot of ways to tweak each build based on level, equipment and playing style.
Conclusion
By late game, most players might find the 2-H Grissenda the most effective companion due to her high-damage, stun chance and still decent armor. However, a well rounded Grissenda or agility focused Grissenda, offers the optimal balance as a player progresses through the game to reach end-of-game levels.