no difference, but there's always a special impression when seeing a 512 image than seeing just a number. u know more details, more color impression and etc. using the 512 as main preview and putting other sizes as just numbers is even better. not tmuch important, just an offer .. ..
Nice icon but I have to point out that in real world, the color combination of your icon is impossible. The rule is for each colour, there are 9 patches: 1 in the center, 4 in the corner of edge and 4 in the middle of edge. In your icon, you have 5 blue patches in the corner of edge.
And another question, where is the 512X512 version? Without it, it can not be a full collection.