Here is an interesting experiment a guy did to see how accurate torque wrenches really are:
http://home.jtan.com/~joe/KIAT/kiat_3.htm
According to his tests, both types of wrenches are accurate if used properly.
I personally prefer a "clicker" over a beam wrench. I find a beam wrench a PITA to use in a place where you just can't read the scale. Even when there's ample space around, you always have to bend over into an awkward position to read the scale, while applying force. With a clicker, you simply position yourself the most comfortable way and pull until it clicks.
For those of you using a clicker, make sure you always return it to the lowest setting after use, but DO NOT go past the lowest setting or parts may fall out of place inside the wrench, requiring that you disassemble it to put the parts back where they belong (don't ask how I know...).