better in timpb than tim

I keep wondering why TIMPB is not a better buy than TIM. In short, TIMPB gives you 8.33% fully franked return plus return of $2.05 value in Sep09 versus share price of $1.62 today. This is a capital gain of 26.5% or roughly 19% per annum. So total return is 8.33% per annum ff dividend plus 19% per annum. So you can gross up 8.33% to compare to other investments = 12% per annum dividend plus 19% per annum = 31% total return. Risk is you get shares handed to you in Sep09 which you must sell so if everybody sells at same time, TIM goes down. But this risk can be managed by CFD once we get to 20 days VAWP. Any views?