If it fell more historically then it is intrinsically more risky.
On interest rate risk, the two funds will be very similar. (short term bonds with higher coupons, ie the first fund, have a marginal advantage when interest rates rise, but I wouldn't depend on that, it could only be theoretical and overwhelmed by other factors (extension risk on Mortgage backed securities, illiquidity, general market forces on dealers).
On credit/ default risk, you'll get a much bumpier ride with the first fund. During another credit crunch, the market 'flight to safety' drops everything but the safest bonds (US Treasury Securities).
If there is a standoff re debt limits, weird things happen, but there's no reason, to my knowledge, to believe that US Treasury security holders will not get their money and interest back in full. To say any more risks a political discussion which is not my intent.
As well as the historic performance, the higher yield on the former fund tells you that it is more risky. The long term perforrmance of the more risky fund is pretty similar to the latter fund? So you did not really get rewarded for taking on that risk.
If this is your Emergency Fund you want to be very safe. I would also consider laddered CDs (within FDIC limits) because they may be paying superior interest rates. I believe there are cashable CDs (with an interest penalty).
Also as I understand US I bonds (WARNING NOT USIAN, THIS IS NOT ADVICE, THIS IS HEARSAY FROM AN INEXPERT OBSERVER) they are not instantaneously cashable unless owned for at least 5 years?