The 6GB/S models cost more. Rather than lower the price of the 3GB/S models, they simply replaced them. You can still get the older ones and they are cheaper but they are rapidly vanishing.
I have OCZ Vertex IIs in our Intel Mac minis. They are older models and wouldn't take advantage of the 6GB/S SSD models anyway. I would think the newer mini would use the faster drive but I would check before spending extra to get 6GB/S only to find the motherboard in your mini only does 3GB/S.