Electronic – Output voltage of MOSFET differential pair with current mirror load

If \$V_{GS}=V_{DS}\$, and \$V_T>0\$, you can change the saturation requirements of \$V_{DS}\ge V_{GS}-V_T\$ to \$V_{DS}\ge V_{DS}-V_T\$. Subtracting \$V_{DS}\$ from both sides gives you \$0\ge-V_T\$, which can also be written as \$V_T\ge0\$. This is why this configuration is always in saturation as long as you meet the other saturation criteria of \$V_{GS}>V_T\$.

There's no need to use a resistor between MCU pin and FET gate.

The 8721 quotes a Vgsthmax of 2.35v at 25uA drain current.

The 44 quotes a Vgsthmax of 2.0v at 250uA drain current.

Both appear somewhat marginal for a good conduction current at 3.3v, though the 44 is clearly able to conduct more current at a lower voltage, so you're more likely to get away with it using this one. Depending on how much current you want to sink, don't be surprised if they don't turn on fully, and get hotter than you expect.

You might be better with bipolar transistors, they are easy to turn on fully with 3.3v. Here you would need a series resistor to limit the base current.