Both Gordon and I agree on the use of the full sinking(Sunk for the Brits and Scots)lines. The down side of the Sunk Line technique is that you have to master the drift more diligently in slower and not so deep water. However,there are some very sound points in using dark coloured sunk lines..the visibility factor from above(Fish Eye View) and the way the line actually tracks the fly so naturally. But you must be careful to manage your line's drift so as to not wrap around underwater snags.I have also found that when mending the full Sunk line there is less irratic impact on the fly itself during the drift from a mend...thus sight to fish is more constant! BTW,the full sunk line we(?) are talking about is actually a shooting line...like the Hardy Mach 1 ,Snowbee Scandanavian Short Head Spey and others where the head is about 45' long with a lighter coloured running line .The Scandanavian's have been doing this for some time,what we are also calling "Scando Style". Loop has a great selection of shooting line set-ups for Scando and Underhand Styles.
The Skagit system is entirely different...a large balloon body(27') that floats, having an attached tip of various densities. What the large body of the Skagit does is allow you to create short stroked casts with little effort,and at the same time pick-up big flies and heavy tips that are not too far down in the water column. So as you can imagine,with a 20' length of T14 off a 550 grain Skagit line on a 14' Two Hander,a simple and well timed Static pick-up will lift the entire line to the surface whereas you can then continue to complete the desired cast. Rod length is somewhat important, but I often will use a 10'6" Switch rod with a Skagit set-up,shortening the tip length and increaseing the timeing of the pick-up and forward stroke. But with the shorter rods you must be very clear about rod position and stop location..there is a tendency to bring the rod back to an almost vertical location off the shoulder resulting in the loop formation being directly behind body and ' Ouch"! on the forward stroke.
Have fun
C