Saturation Points for Mosfets; driving gate with 3.3V output

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.

The gate threshold voltage is pretty much irrelevant to most applications. Note that this is usually defined where the FET conducts only a little current. That's not useful when you want to use it as a switch to conduct lots of current.

The main specs you need to look at are R_dson, and at what gate voltage that spec applies. Things you do with this:

1. Multiply R_dson by the maximum current when the switch is on. This tells you how much voltage the FET will drop. Make sure that amount of voltage loss is acceptable to your load.

2. Multiply the square of the current by R_dson. That is the power the FET will dissipate when on. Make sure that is within what the FET can do, and that your mechanical setup can deal with the heat.

3. Make sure the circuitry driving the gate guarantees at least the gate voltage that R_dson is specified with.

If any of the above don't check out, you need a different circuit or a different FET.