I agree with Greg and here are my thoughts.
I bought both. I use the 16 KG for rep work and the 24 KG for 4 to 6 reps on swings, snatches, C/J and presses. I can also snatch 55 LB DB's for moderate reps fairly easily so I was surprised by the difference when using the KB's. I find it slightly more difficult with the same weight KB.
If I was to only buy one it would be based on what I was trying to do. If you are looking to build strength endurance I would definitely go with the 16 KG. You can do all the same exercises for reps and get a great workout.
What we think, or what we know, or what we believe, is in the end, of little consequence. The only thing of consequence is what we do. -John Ruskin