I couldn't sleep last night and ended up wondering if you'd make more money over 15 years if you made 1% interest the first year up to 15% on the last year or vice versa, making 15% the first year and 1% the last.
So my gut tells me that making 15% the first year will yield more money as you're making interest on a larger amount (after the first year) and compound interest loves time.
Maybe you'll end up with same at the end but I can't imagine having an incrementing interest rate will yield more than a decrementing one.
To the spreadsheets!