Neither is good for sneak attack; at range, you cannot flank, so you rely on catching opponents with their Dexterity to AC denied. That can be difficult to rely on. Before any other consideration, you have to determine how you are going to be doing that on a regular basis, while still actually having the time to attack.
Both options do have the upside of their large numbers of attacks (Rapid Shot, Greater Manyshot, splitting weapon property), which lets you apply sneak attack repeatedly once you have enemies without Dex to AC.
As for which one to ultimately use, it’s largely a question of how many feats you have available. Bows are much, much (much, much) better than crossbows out of the box, but there are a few crossbow-only options that can make crossbows better. You have to figure out, basically, if you have enough feats to actually overcome bows’ natural advantages (free reloading, Str to damage). Remember anything you invest in fixing crossbows is something you could have invested in improving a bow.
- Rapid Reload mitigates the biggest problem (by far) of crossbows. It isn’t enough to make them worth using, but without it or something similar they are definitely not.
- The ghostly reload spell from Races of the Dragon can replace Rapid Reload, probably. The spell is kind of unclear: it says it will pull the string back into place automatically, but does not clarify what it takes to then load a bolt into place. Anyway, this spell is 1st level and has a decent duration, so it could be reasonably used in wand form, though you’ll need a decent caster level on it and that will get pricey. Wand chambers from Dungeonscape would make such a wand more usable. But going for an unseen seer/arcane trickster build would be far better.
- The quick loading weapon property in Magic Item Compendium is a +1-equivalent that stores bolts in an extradimensional space and can automatically load them into your crossbow as a free action (hand or light crossbow) or move action (heavy or great crossbow). Ask your DM whether or not quick loading and Rapid Reload stack for the bigger crossbows.
- Crossbow Sniper from Player’s Handbook II allows you to sneak attack from up to 60 feet away, and also allows you to add ½Dex to damage. Requires Weapon Focus (which you also require for Dead Eye from Dragon Compendium for another +Dex to damage). The ½Dex to damage can easily be bigger than the Str to damage on composite bows.
- Great crossbows from Arms & Equipment Guide have really high base damage. Like, almost-actually-worth-a-feat good. But see below: you may not even have to spend a feat on it.
- Hand Crossbow Focus from Drow of the Underdark gives you both Rapid Reload and Weapon Focus with hand crossbows in one feat. Note that Rapid Reload is crucial, and Weapon Focus is necessary for both Dead Eye and Crossbow Sniper: this is a very, very good feat.
- The aptitude weapon property from Tome of Battle lets you use the weapon proficiently without actually having proficiency, and lets you apply feats meant for other weapons to that weapon. So you could, arguably, get a +1 aptitude great crossbow, skipping the need to get proficiency in great crossbows, and then apply Hand Crossbow Focus to it, allowing you to reload the great crossbow as a free action and getting Weapon Focus in it. Note this is some seriously borderline abusive stuff here; aptitude is a rather ridiculous weapon property.
- Hand crossbows can be dual wielded, which no bow can do. Though archery feats + two-weapon fighting feats + feats to make crossbows not suck is more feats than pretty much anyone has. And on top of that, you have to figure out how to reload with both hands full.
- The gloves of the balanced hand from Magic Item Compendium can give you Two-Weapon Fighting, or Improved Two-Weapon Fighting if you already have TWF. Per the same book, they can also be gloves of dexterity at no price premium, just the cost of the two added together.
- If you have four arms, great crossbows can be dual-wielded too. Just saying.
- The spare hand from Magic of Eberron can reload crossbows for you.
- Gnome crossbow sights from Arms & Equipment Guide let you treat targets as if they were two range increments closer than they actually are. Whether that makes them actually count as closer for all purposes (read: for whether or not you can sneak attack them) or just for range penalties is unclear; ask your DM. Since they’re a mundane improvement that costs just 150 gp, get one either way.
In the end, a four-armed monster dual-wielding a pair of +1 splitting aptitude great crossbows with Hand Crossbow Focus, Dead Eye, Crossbow Sniper, Rapid Shot, and the Two-Weapon Fighting feats and a spare arm is looking at adding 1½Dex to damage on each of fourteen attacks; sneak attack is, at that point, almost an afterthought. Consider dips of barbarian 1 (whirling frenzy gives another attack, which splitting of course doubles), fighter 1 (hit-n-run tactics from Drow of the Underdark offers Dex to damage, again, against flat-footed foes), and windrunner 2 (from Races of the Wild, windrunner gets a primal scream that gives a huge chunk of Dexterity, effectively +9 to damage). The Craven feat is also going to add a bunch of damage. But that’s also an absolutely ridiculous number of feats, plus you have to figure out getting four arms (Savage Species has a psionic-less thri-kreen that’s only LA +1? and an insectile template for six arms; tri-wielding?)