There are different schools as to what length circs you should get. Some, as you have seen, say 2 different lengths, some 2 - 24", some 2 - 16"; all in all, it's what you are most comfortable knitting.
My preference, 2 - 24 in. circs. I prefer this length, the actual needle part is longer than on the 16" circs.
When I 1st started buying circs for socknitting I bought 16" and 24" so I'd have each length needle but quickly discovered that I was more comfortable knitting with the 24" circ, so i bought more 24" circs for socknitting. Shoot, when I couldn't find 24" circs for my socknitting I bought 32"!
All that to say....2 - 24 in circs for me