I have been using the HP15c (did I tell you that I have one for sale?) during this last two days, after some time of being using the HP41CL. In general terms, I prefer the latter, due to a better keyboard, a more informative screen and a more comfortable viewing when on a table.
However, when you’re traveling, the HP41cl is heavy, you need to pack it in your suitcase, and you risk having it damaged.
Instead, the HP15c can be carried in your jacket pocket, and you can use it in the airplane without resorting to your suitcase. I have to fly a lot lately, and i can use it this way. Incidentally, this can be done with the HP42s too - it is just that I prefer the “landscape” orientation for using the calculator with a single hand (typical when you’re sitting in a low cost airplane seat!) Apart from that, I got used to the better screen contrast of both the HP41cl and the HP15c, and then, reading the HP42s with bad light conditions and a worsening sight is tiresome.
But next week, I am returning to the HP41CL! I will investigate modules with statistical distributions.