User blog comment:YourPrivateNightmare/Hail the hourglass!/@comment-4921386-20130914223152/@comment-8550255-20130915050927

well I think the logic is about buying GA...Both just grants you invincibility and also has seconds of not doing damage.

Having Hourglass means:

controlled invincibility, has less CD but comes with useless AP, no MR and is easily become useless against preemptive kills like Rengar Dives.

While GA has

Automatic invincibility, have MR but becomes extremely useless when on CD.

I think they only have very little gold difference (like 400g) assuming you build these items as your last item on an ADC (or maybe 5th).