Generally you can't have the best of both worlds!
Once the 15" Alu comes out, the 15" Ti will be no more straight away. Production will have usually stopped weeks before to ensure that the channel is clear.
Apple will not offer old and new models for sale together at different prices, it is just not in their interest to do so. (However there may be some Ti's left in the refurb store).
It is most probable that the price of the 15" powerbook will go up...
YOu have a final option of being able to buy a 15" Ti from a reseller somewhere... they usually have some of the old models left when new ones come out.
I will say though, that all this is speculation, and I have learnt not to second guess Apple! It could be months before a new 15" Alu comes out. It could be hours...
Buy what you need, when you need to.
Peace,
Marc