Ideally who wouldn't want both. However, if I had other gear to at least somewhat compensate for the shortcomings of either of the lenses, then that would be reason enough to not have to buy both. Basically my goal is to get maximum mileage with minimal gear while at the same time not compromising quality to a significant degree. I do agree that its a battle between IS vs F-stop but I can easily think of pros and cons for both and honestly I think I could live with either. Lack of either will produce just as much disadvantages given the scenario. I'm not trying to diminish the purpose of either, but if I had the option to buy another lense instead of getting both of these, then I think the former option is a lot more useful to me in real world application. If you think of it that way, then overlap of focal range and things like distortion are strong considerations if I had to consider "is it worth getting both or do I spend it on another lense".
i dont think i've suggested to you to buy both lenses.
also, i think at the end of the day, you'll talk about bokeh, weight and other factors before something like distortion of that 2 of your lenses overlap partially. heck, i might even say ditch both and get a 50/1.4 or a 30/1.4 [if you decide to stick with 1.6x crop]. it'll give you nice, sharp photos that pop. this will fulfill your minimal gear philosophy easy. but of course, me thinks you're going through a bit of analysis paralysis and are starting to become one of those perfectionists who worry over even the minor factors when considering a lens.
if image quality is #1 no matter convenience or IS, i'll say 24-70 hands down. better bokeh, more pop. hopefully this fits you. otherwise, u gonna have to sell it and buy the other one... which is a small hassle

[edit] i think if i go absolute minimalist with gear, i'd get the 24/1.4 L, 50/1.4 and the 135/2 L and maybe an xti to use as a makeshift extender and 2nd body