I've heard both but it's your ears and it's your money, audioman. The tweeter isn't the only upgrade according to PMC - also the crossover and the cabinetry. Another consideration is that you can already see the difference in resale value - that gap isn't going to get any smaller. People are asking ( and paying ) more for the i-series for a reason, you'd think. Also, is it the FB1 or the FB1+ which you are comparing against the FB1i ? If it's the + series, I believe it has a silk dome tweeter ( don't think it's the same as the one in the FB1i but could be wrong ) but if it's the FB1, it's got a metal dome tweeter and that's a very different animal. Make sure of which model you're actually looking at.
D.D.