I also have 15C from my early University days and my 28S from graduate work, both still working. I got the 15C used from a junior EEE student that was quitting the program. I think it's on the 3rd set of batteries in 35 years. It got used several hours a day for the first three years.
The 28S burns through three N batteries every two years, even when it doesn't get used much. That may have contributed to the battery doors wearing out on the 28S. Mine is still in great shape however, as spent most of it's life on a desk rather than thrown in a backpack with heavy books that the 15C endured with aplomb in it's case.
The 28S was at work and the 15C at home. Now that I'm working from home and they both are on the same desk, I find I pull out the 15C unless it's complex enough that I want the four line display on the 28S. And if it's slightly more complex than that, a spreadsheet is probably the better tool.
The 28S burns through three N batteries every two years, even when it doesn't get used much. That may have contributed to the battery doors wearing out on the 28S. Mine is still in great shape however, as spent most of it's life on a desk rather than thrown in a backpack with heavy books that the 15C endured with aplomb in it's case.
The 28S was at work and the 15C at home. Now that I'm working from home and they both are on the same desk, I find I pull out the 15C unless it's complex enough that I want the four line display on the 28S. And if it's slightly more complex than that, a spreadsheet is probably the better tool.