As revealed by Nintendo Gamer, FIFA 13 on Wii is nothing more than the Wii version of FIFA 12 with the number changed, transfers updated and kits changed. They've posted up a lot of comparison screenshots, I'll quote some of them below. On the left, FIFA 12, and on the right, FIFA 13:
More screenshots at the link
This should have been a free update, or at the most paid DLC for a reduced price. Charging full RRP for this is an insult to Wii owners and anyone who buys it.