I'm getting ready to move to a 4 inch piped (sheet metal snap lock) duct system with drops to replace my single duct moved machine to machine approach. Contemplating it, I have some challenges that require some of the connections to be really flexible (I have to physically rotate my on wheels collector about 30 degrees to get at my clamp rack and I have to move it about a foot to change my air cleaner filter). I can use 4 inch flex duct for those, but it put me to wondering about the comparative disadvantages of flexible duct over adjustable elbows for the rest of the run. Using flex duct I can get a r-e-a-l-l-y long bend which I understand helps with losses. On the other hand an adjustable elbow has smoother sides but I'd have to use two of them to get a long bend. I presume the sheet metal approach delivers better performance, BUT IS IT A HUGE ADVANTAGE OR DOES IT MATTER ENOUGH TO WORRY ABOUT IT on a run that has only three bends in it and one of them is only going to be about 45 degrees? Total run length across the ceiling is about 20 feet and then add another 10 feet for going up and then down the wall for a total run of about 30 feet of duct. Thanks for your thoughts and experiences.