When it comes to attaching my fly line to a leader, I like the streamlined profile of the nail knot. I've been a nail knot guy for years. I think it helps the line and leader run through the guides with less resistance. This is particularly important during the end stages of landing fish, when you crank some leader past the tip.
I also trust the strength of nail knots more than I do loops. Over the years, I've had 10 loops break to every one nail knot that's failed. So now, when I buy a packet of pre-looped leaders (or a loop end fly line), I often cut off the loops and use a nail knot tool to tie the leader on.
But I'm starting to change my mind. There's just something to be said about the convenience of the loop to loop connection. Attaching a pre-looped leader to a pre-looped fly line takes seconds, not minutes, which can actually matter in the heat of the battle. I also think the manufactured loop ends on fly lines are at least 100 times stronger now than they were 15 years ago. I don't know exactly what happened, but most manufacturers seem to have figured it out.
I also have many friends who insist that a loop-to-loop, done right, is far stronger and more reliable than a nail knot, especially on larger fish. At a certain point, when you're using heavy leaders, a loop has always been the natural choice. For example, when Oliver White showed me how to catch the arapaima in Guyana, every rig was a loop connection.
Still, in the context of 4X leaders for trout fishing, I'm on the fence. You tell me, please, what's best?