First, let me start off by saying the most important deciding factor is personal taste. If you love using one over the other, don't change your configuration and enjoy playing the game less.
Additionally, weapon DPS the ultimate deciding factor. Thus, this guide assumes comparing weapons with the same DPS.
With that out of the way, let's look at the various properties we need to consider.
Special Mechanics to consider
The damage spread on Hand Crossbows is larger than the damage spread on Bows or Crossbows, on the order of a 50% damage spread VS a 25% damage spread.
Generic Weapon Properties
These properties are only found on hand crossbows and Quivers; Crossbows and Bows do not have these properties.*
*Note: Some legendaries, such as the Windforce level 60 bow, do have +Hatred Regen. Given the rarity of this item and the lack of lesser Rares and Magic items with DH specific stats, we will continue to use this assumption.
Passive Skill Choices
See this post on how figure the Archery passive into this decision making process:
Given the above, we can make the following conclusions.
2xHand Crossbows is superior when:
It's worth considering the Time To Kill metric of most enemies; consider the following example: A mob's health is reduced by 50% when struck with a hand crossbow. It's reduced by 90% when struck by a regular bow. The bow did more damage, but both weapons will need to fire an additional shot to deliver the killing blow.
Additionally, the faster a weapon the faster the recovery time of firing it. Therefore, if you need to use vault or whatever a tenth of a second faster after firing your weapon then faster weapons come out on top.
Furthermore, using 2x1H weapons means sockets on both weapons count as a 'weapon' slot. A 2H weapon + quiver counts as a 'weapon' slot and an 'other' slot for the purposes of socketing gems.
Finally, abilities like Bola Shot + Impale have a higher chance to keep a boss stun-locked when using the faster 2xHand Crossbows setup.
In conclusion, if you can find the correct stats on Hand Crossbows (proc on hit, +hatred regen and + max disc), then Hand Crossbows are better than 2H bows and crossbows.