I would absolutely get the IIa for the collectibility factor of just knowing I had one. BUT barely anyone you encounter will know what that means. Since I am mainly into colored gems, I have a lot of things that people have never heard of and I am not bothered by most other people not knowing what it is that makes a particular stone neat. They're called "collector stones" for a reason - the other people who will know and be impressed are also collectors or dealers. So if it is just for an engagement ring, for someone else, or you're sensitive to random people's thoughts on your jewelry, then go with the bigger one, since that is what people will see.