Good point FTR, I suppose it does depend on how tall you are, I'm 5'2 as well so lowering him in without waking him was difficult!
You drop the height to the low one when they start standing up or look like they might climb over it, so depends on the baby and how active they are. That's why I think the ones with the drop sides are better (I wish I'd got one). Also depends on how deep the high up setting is, ours was about mid way down the cot but my friend had quite a shallow one so she'd have needed to drop hers down sooner.
Also, most babies seem to outgrow Moses baskets at about 4/5 months, and even before that you might find you use the cot a bit during the day, especially if it has a cot mobile on it, because its another space to put the baby in to keep them happy and keeps them interested. You might not use the bed nest every time you want to put the baby down when it's awake, eg sometimes you'll put them on your bed, sometimes the sofa, sometimes a play gym etc, so you might use the cot earlier than you think.
Ah, the trials and tribulations of cot buying! Have fun!