Just a point to note. Heckler is not the only "Fast" crossbow. There is also the Maple Crow, which adds speed as well. Also, I don't recall a "Slow" xbow. Most are either "Fast" or "Normal" attack speed.
Personally, I've handled both bows and xbows. Not much difference for me. Remember, xbows get Booster as well.
I recall a thread with this exact argument by Julliant(?) comparing the speed of bows and xbows. Here you go. Search FTW!
Julliant:
http://www.hidden-street.net/forum/s...ad.php?t=24662
ElderDragon:
http://www.hidden-street.net/forum/s...ad.php?t=23989