I'd wager a fair amount that the price of the Alu 15, when it finally gets released, will not under any circimstances be lower than the Ti 15 current price...
Loads more features, new machine casing, more R&D etc... prices may even go back up a bit. Who knows? All is speculation and supposition...
As to what to buy... if you need a machine right now, buy the Ti. If you can wait, wait.
Either way you get a great machine...
Peace,
Marc