There are pros and cons to both types of seat clamps.

 

Bolt Style Seat Clamps

Bolt style seat clamps are good for people who will be leaving their bike somewhere unattended, as it is simply a matter of undoing a QR and stealing your expensive Thomson seat post and Brooks saddle you have on your bike.  $200 gone in the blink of an eye.  Also torquing a bolt with a torque wrench is a handy feature to have, especially if you are running a carbon seat post.  Weight weenies will also appreciate the lighter weight of a bolt style vs. a QR. Generally speaking bolt style clamps tend to be cheaper, making the penny pinchers happy as well.

Quick Release Seat Clamps

QR style seat clamps are especially good for people who do a lot of downhill, or have big drops so they can lower their seat prior to tackling.  If you are the type that likes to switch up bikes, with riding buddies, for a loop or two, QR seat clamps make it a snap to adjust for each rider. Finally, for riders that like to switch up their seats and/or posts between street riding and trail riding, a QR style clamp is handy option to have.