I highly doubt the prices of the 2008+ STi's will drop that much. While that may make sense for some brands, the STi's are usually hard to find (both new and used) when compared to most other vehicles. As a result, the prices usually will never fall that much...for example, look at the wrx over the years.....a 2002 WRX with over 100K miles will still sell for over $10K....that's only half of it's value lost. Normally cars with over 100k miles will sell for around $4-$5k (if that).
I would say that unless you are really going to use the things the STi offers over the WRX (which really is only the Si-drive and DCCD) keep your WRX and mod it a little. All you really would need to do is tune the rex add some coilovers and put better brakes on it and you would essentially have a STi.....the tune will put it very close to the sti in power, the coilover would actually allow the rex to handle better than the sti and brakes will help some, although not enough to really notice on the street....it would more just help with looks.
Doing all this would save much more, than trading/selling your rex and then buying an sti....oh and the nice thing about modding the rex is that when you go to get a new car in the future, you can easily sell the parts you buy and get a decent amount of your money back which would make modding the wrx even cheaper than buying the sti.
I not only sell Subaru but am a huge fanatic and imho, the STi is really not the best value for dollar when compared to the sti....is the STi nice? Sure! Does it have cool stuff on it? Absolutely! But is it worth the extra $8K-$9K or so? I don't think so....because you can add the same stuff to the rex for less (with the exception of the dccd....which a majority of the people don't really know how to use appropriately).