And if inflation and interest rates go up, the I-Bond interest will go up, too. The CD interest will stay put, and if you need to sell before maturity you'll take a hit on the value of the CD.
Not so much of a factor with CD's bought directly. If you sell before maturity (because you need the $$ or want the higher rates) the lost interest typically ain't much at present rates.
I-Bonds seem to be an okay place for fixed income allocations today. But let's all make a pledge to nag each other to climb aboard if we ever see the I-Bond fixed component at 2%+ again. I've got some at the old 3%+ fixed component rate, and the only thing bad about them is I doubt I could ever make myself sell them until they hit 30 yrs.