I've flyfished for 40 odd years and this thread has made me realise that I only possess one DT line now. I far prefer WF for my fishing (90% small streams and small rivers, 10% stlilwater) as it casts better , and by that I don't necessarily mean further, but faster, with less fish spooking false casting. Presentation is just as good as with a DT with the right technique , helped by the fact that I'm using 3 and 4 weight line.
I use DT on one rod as it cast like a bullet with a DT 4 , but terribly with a WF. It is the only rod I own which feels different with DT from WF , illogically or not .
For stillwater, troutfishing I'm using a WF5 on small lakes , and casting 20-25 yards is very easy with practice , on the rare occasions it is needed . Presentation is good , but I'd struggle with sizeable carp if snags were a problem, I catch rainbows of up to 7-8lbs easily on this rig ,using leaders of 5-7lbs bs, but although rainbows do fight very hard , with long runs , they don't fight smart and don't often run into snags or weed deliberately .
I use a 9-6 #7 weight for big lakes , where wind is an issue and casting 25 yards with a single false cast becomes routine. I'd feel confident playing carp , or pike , on this rod with no problem . If I were flyfishing for carp I'd use this with a 12-15 foot leader of 10-12lb bs. The leader is longer than many use , but I habitually use longer leaders than most as it gives better presentation and , with the right technique, casting is easy . For stillwater trout I often use 18-20ft (to fish with 2 or 3 flies ).
Flylines are the preserve of the snake oil salesman , with silly claims being made about how brand A gives you 8.2% more distance than brand B ,and it's almost invariably nonsense . I'd advise any beginner to get a 20 or 30 quid line, or cheaper, but buy some casting lessons . Ignore all those silly diagrams in the magazines about the sort of loop you throw , as nothing beats practical hands on experience and guidance to perfect technique . Confidence is key , and to create it the building blocks are timing and decisiveness -no shilly shallying with hesitant , and too numerous , false casts !