It is probably worth rethinking a bit the conventional wisdom regarding bonds as most of the theory around AA is written during a time when bonds actually provided yield, and even some real yield on government bonds, which is now negative all across the board. Im not saying it's wrong to hold long-term government securities, but it's important to understand the risk one takes by buying bonds with long duration - credit risk and credit spread risk if corporate bonds and rate risk for both corporate and government bonds. If, for some reason, interest were to go up and/or you get inflation those securites will be abseloutely and utterly massacered and mor the longer the maturity. There is no reason why this has to happen (higher yields / high inflation) but if it does, it will get very ugly. Yes, when you lend to uncle sam you will get your investment back at maturity, that's the zero credit risk part of the picture, but what those dollars are worth in real terms when you get them back might be a completely different story.
For the safety I would lean towards treasuries with shorter maturities as for me the risk/reward of holding long-term bonds isn't worth it. It might turn out somewhat better in the end, but it might also be a lot worse. Credit spreads are low these days, so the extra payoff you get from taking on credit risk is pretty bad.
What long-term bonds has and especially when yields get low is convexity. The value of bonds go up quicker than it goes down when interest rates move. If we take the current 30y US treasury it has a coupon of 1.625%, matures on the 15th of November 2050. It currently trades at a price of 100.06 and yields 1.62%. If yield doubles to 3.24% the price goes down to 69.16, so a drop of around 30% in value. But for an equal move the other way - yields go to zero, the price goes up to 148.74 - a gain of almost 50%.