Since there is not so much here to confuse anyone, I'll add my .02 worth. Floating hat is correct in that (most) poly leaders, being lighter weight than a full fledged sink tip, cast nice, get down good, but lack the mass to turn over big stuff. Case in point: I have a Rio 15 ft type 8 sink poly leader that weighs 58 grains. I also have a Rio 15 ft type 8 sink tip that weighs 150 grains. The 150 grain tip is Tungstin coating built on a 35 lb mono core, so technically you could call it a poly leader. Both being type 8's, they sink at the same rate. Both being 15 ft long they will stay down throughout the swing pretty much the same. The 150 grain tip will turn over the average intruder type fly. The 58 grain tip, no way. However, and this is important, the 150 grain tip will not turn over even a small fly if the fly line it is attached to tapers down to less than .070 diameter at the tip! In fact it will cast like sh!t! It will not even turn itself over! Well, if you put a lot of power into a very short cast it will. But it ain't pretty.
Most poly's come in "trout" and "salmon" sizes, which have breaking strengths of something in the neighborhood of 10lb and 20lb. This relates to the mono core they are built on. Some can also be had in varying line wt designations, such as that 58 grain tip, which is for a 7 wt. (I think) And a lot of the lighter ones will cast reasonably well looped directly to the tip of a fly line. Like someone said, find what works for you and then fine tune, fine tune, and continue to fine tune.