Bills are shorter duration, bonds are longer duration. You can look up the terms to get exact time ranges. When the yields for both are very low, there's next to no upside to holding onto the low yield for longer. The downside is that if one is in critical need of cash immediately, nobody wants the old low-yield (compared to current yields) bonds and will only buy them at a discount. For bills, due to the shorter duration, extra liquidity is built-in.