The parallel slide gate valve has a parallel disc sealing against seats by means of spring force, whereas a wedge gate valve has a wedge shaped disc which gets locked on to the seats by means of the mechanical seating load applied. Where a very tight shut off is required generally wedge gate valve is preferred. But in this valve the body cavity may trap pressure and hence particularly in steam applications can result in locked-up condition of wedge. Hence, in such applications a parallel slide gate valve is preferred.

I believe that some of the parallel slide gates generally port off the high (as I recall) pressure side to the area between the gates to provide a tight pressure seal (equalizes the high side and allowing the springs to work)of the gates... this type of valve can lock up particularly in steam applications IF this center area is not connected (and allowed to vent through the ported connection) -- these type valves will usually have a small pipe on the outside of the valve connecting the center portion to the high side...

The relieved "bonnet" area is fast becoming the standard for PSVs. It is my belief that while wedge gates still have their place in industrial applications, steam is not one of them. As expansion and contraction occur a forced fit wedge tends to bind. This is particularly a concern in Combined Cycle plants where temps are going up to 1100F.
     Another concern for this is training of the users. Operators are brainwashed that to get tight shut off a valve must be wrenched into the seat. While this is true of a wedge gate due to a forced or "torque" seat, a parallell disc gate is position seated. With a Parallel gate if the handwheel is hit with a cheater bar the position bar will bend. You should look for a valve that has immovable stops cast in the body to prevent this.