I keep recommending 50.
The argument that it drains batteries is to my belief false, mostly because of all components in a PC the processor consumes the least. We could better take off the real killer, the screen. Just remove the backlight and add in the juice. I know a calc isn't a pC but just saying proccesor consume little to no power.
The argument that it costs more is also 'debunked' price difference is ~$5
So please vote for 50mhz! It adds greatly to resale value!
It depends of the country. Last time I checked, rechargeable batteries costs around $13 for 2 batteries here. The charger costs $23 and that's if you get the crappiest one. Now if you are a programmer, you need at least 8 batteries (assuming the calc uses 4 at once and you use both sets every day). That makes it $75 total. If for $1.68 you can get 12 cheap disposable batteries and each set of 4 lasts two weeks, after 5 years you spent $50.80 on batteries while with rechargeable ones, you would have spent $75. I gave 5 years as example because rechargeable batteries lasts around 5 years with heavy usage and almost everyone stop using their calcs after 5 years.
So for some people, that $5 would be more like $25. Note that I include taxes in my prices, though. I also live in Canada (Quebec)
Regardless, if the calc can be set to support two CPU speeds, it would be fine, I think. If somebody programs a game, for example, he should probably set it to run in 20 MHz mode for menus and if it's not a very intensive game, I don't think 50 MHz is necessary at all. For math operations, 20 is enough for almost everything except graphing .
Also I changed my vote to 50 MHz.