I find clipless is easier on my knees also(especially the more sensitive one), cos i spread or alternate power that goes on pedals, with platforms it's narrower range to apply power (mostly downstroke).you are little more relaxed/don't waste tiny energy in keeping your feet on pedals so it helps with endurance. for efficient pedaling clipless is the way to go, but since im new to clipless (one month) i still find loose gravel on steep descends and ascends, safer on platforms..