I would also base it on the usage level and data types used. Video, pics, music data sets can be very large and consume huge amounts of fresh blocks. These things are like coffee pots in a way and larger is better since you can serve quite a few more cups of "joe".. before you have to make more(GC/recover the drive).
With current firmware, capacity is the greatest defense against degradation. So, in the end you need to base it on the intended usage level and how much you're willing to spend. Otherwise if you chintz out too much?.. you'll need to modify your usage and maintenance schedule to compensate for too little capacity.