I wonder which one I should use for Imoen? Which one is the better? Or are there any better bows in BG 1?

Before you equip weapons on anyone, you should check their proficiencies on the character screen. IIRC, Imoen has Short Bow proficiency, so you should use that on her.

You should also think about ammunition. There are a lot more to choose from for short bows, for example arrows of dispelling, for which there is no corresponding ammunition for crossbows.

Another consideration is the highly powerful Firetooth crossbow at Watcher's Keep. Simply have enough gold and the best crossbow in the game is yours. You only need to show up at any point in the game, and purchase it. You need not complete the quest for the Imprisoned One until later in the game.

Finally if you have Jan in your party, he takes care of your crossbow needs handily. His special ammo (bruiser mates) is very handy early in the game.

Attacks/round are KING. A short bow (without any special enchantments like the Tiguan Bow from BG2) will yield 2 attacks per round on any character without any proficiency points in that kind of weapon. I'd be flabbergasted if the Light Crossbow of Speed did not grant you a higher number of attacks per round (iirc, it's set to three). Just use the one that makes your char resemble a machine gun more closely!

iamdavid is right, it depends on proficiencies. Imoen starts with short bow proficiency and you can add one level to missile weapons (crossbow) as she levels up in the game. She's only allowed one level in each, so the difference is rate of fire and personal preference. Light Crossbow of Speed gives you one extra attack and a +1 bonus, but it is expensive. You probably won't be able to afford for a while. You can buy a shortbow +1 for less money, and I'm pretty sure there is a shortbow +2 late in the game.

Long story short, if she doesn't have a slot in crossbow, don't bother. Finally, what looks better for your thief to carry?