I would personally only buy really long cables- like 45" so that I only have to have one set and can just ML everything. But it really depends on what you like to knit and if those are long enough for you (short cables feel really awkward to me).
It all depends on your preferences and what you want to knit. I personally hate magic loop and don't like long cables because they get in the way. Plus I knit a lot of soakers, so I needed the shorter 16" needles. They were all I used for a long time, and I used them for everything I knit. Now I have an interchangeable set, and I only have 24 and 32 cables for that, along with my set of 16" needles. I use the 24" needles for things I make for myself (or other adults) and the 16" for things I make for my children, since they're too small for the 24".