I just got finished paring back my 5-speed freewheel from 14-38 (Kansas City hills gearing) to 14-28 (high plains gearing). I had even considered 14-24. BTW, I used Mike Sherman's Bicycle Gear Calculator to decide on the exact gearing - it's fast, and informative of any gaps in the gearing - goodbye semi-log paper and gear charts.
My old PX-10 is set up with Huret Duopar rear and Huret front derailleurs. The Duopar did very well on the 14-38, actuated by a Suntour indexed barend shifter - it works! I also have a Sachs-Huret ECO S rear derailleur, and there is a 24 and 28 tooth position for the tension spring, so it should be good to go on the 14-28. My question concerns the chain-wrap capacity. The front TA crankset is 30-46-50, so does the ECO S have sufficient capacity for that? Maybe someone on this forum can tell me.