In my experience, there's always something new and improved around the corner. That's how manufacturers keep you buying things. The question for you is, how much do you need or want the capabilities of that lens now compared to how much money you have.
If you are a billionaire, just buy both. If you're homeless, probably there are other things to spend your money on. Like most of us, though, you probably fall somewhere in the middle and are concerned about spending a grand on a lens that may be superceded in the near future, or which may fall in price for stock clearance.
In the case of the former, then you'll end up paying even more because goods such as this are always more expensive on their debut. In the case of the latter, well that's just life.
Definitely don't think it is worth waiting for IS because you don't have a tripod. You can pick up a decent tripod for under Â£100, and really good ones for Â£250 or so. A fraction of the cost of this lens!
Ultimately, I find for me that the best philosophy is to put money out of the picture. Either decide you definitely want the MkII, and hence just accept that you're going to pay what it costs, or decide that you want the lens for its parameters and buy the MkI now.
From personal experience, I have had this lens for over a year and find it to be exceptional.