I spend too much wondering how Rosa seems to get the shaft for the best equipment of the game. Her stat growth sucks and that never helps the decision here. Here's a couple of choices and some pros/cons to keep to keep it balanced...
Silence Staff (and Silver/Mythril Shield if you prefer using the Sneak trick)
Pros:
No need to pay for Arrows
Mage killer+Shuts them up if needed
+10 Will to counteract the Heroine/Minerva Robe penalties
(If using the Sneak trick, she gets awesome defense to the point where you don't have to worry)
Cons:
Requires some luck for a Silence Staff drop, but it isn't as hard to get than other drops
Is not "back row" friendly, therefore maximum potential not usually realized
Elven Bow + Your pick of arrows
Pros:
Mage killer
+5 Wis if Rosa needs the extra magic defense multiplier and somewhat counteracts the Heroine/Minerva Robe
Back row friendly
Arrows easily acquirable
Hit rate is maxed
Cons:
Item list sucks with arrows in FF4... much less an issue in FF2/FF4ET
Damage increase may not be too substantial over the Silence Staff (depending on your pick of arrows), even though it is back row friendly
Aim is essentially useless
Samurai/Yoichi Bow + Your pick of arrows
Pros:
+10 Str most likely to tip the multiplier favorably
Back row friendly
Arrows easily acquirable
Cons:
Item list blah blah blah.. then again, Whistles and Big Chocobo are an option
It is kinda mediocre?
Artemis Bow + Your pick of arrows
Pros:
Strongest of all the bows
+10, STR, AGI, VIT are awesome bonuses with a chance to increase some good attack multipliers
Back row friendly
Arrows easily acquirable
Cons:
-10 WIS, WILL hurts a lot for magic defense, especially with Rosa's stat growth... it gets worse when the Minerva/Heroine Robe is involved
Weapon does not live up to its name... nothing special vs Dragons by default
Which version would you prefer?
I don't mention FF4A because there are obviously better options (and Aim actually increases damage output, even when hit rate is 99%). FF4ET makes the Artemis Bow+Heroine Robe combo really really awesome and simply is a superior solution (too bad it isn't used anywhere else).
I'll post a follow up question later...