Johno----I was in a similar situation/dilemma as you until 31 March
. Up until a couple of weeks ago I was planning on holding onto my '04 Z06 until 2012 or 2013 and then deciding to either get rid of it for a Z28 (hopefully with 550+hp) or a C7 Corvette (hopefully similar to the Stingray Concept). There's a lot of unknowns as far as the Z28 (no guarantees on price, power, or when (if ever!). But, an SS now is a sure thing.
After a lot of soul searching I decided to get rid of my Z06 and enjoy an SS for a few years until either the Z28 or C7 comes out. Originally I consideed leasing but leases aren't that great of a deal anymore after the economy took a dump.....at least here in Kansas. I looked into it, and a three year lease on a 1SS was ridiculously high with absolutely nothing to show (think trade-in/downpayment for next car) after those 3 years. I started crunching the numbers, decided I would be better off buying, and after haggling with the dealer I ended up buying my 2SS. Over a 5 year loan, my payments are more than $30 a month CHEAPER for the 2SS than what it would have been to lease a 1SS for 3 years AND when I go to sell or trade it in in 2013, I'll have whatever principal is paid off to apply as a down payment on that Z28 or C7.