This is the only info you need to consider....current sales prices for the VF-31J.....
http://closedsearch.auctions.yahoo.co.jp/closedsearch;_ylt=A7dPFllBs.dYURkAC2yuGDx8?p=DX+VF-31J&aq=-1&oq=&x=33&y=24&ei=UTF-8&slider=0&auccat=0
The VF-31J is arguably the hero valk of Macross Delta...if the price for the VF-31J is close the the current pre-order sales price on these "secondary" retail sources for the VF-31C....then it is not worth buying because, in the very worst case scenario, you will be able to get it at the very same price after it is released based on the current market prices for the VF-31J.....
In short, you can afford to just wait and get lucky, knowing that you will not have to pay any additional amount for it come September.....
The only scenario in which you may want to place a pre-order, "just in case", is if the retail site is not requiring an up-front payment of any kind and there is no "obligation" to go through with the pre-order....