Am I missing the obvious? MOSFET or Darlington?

Why is the TIP120 used so often as a motor speed controller (with PWM) rather than a MOSFET such as a IRF540A? (Or better still - 95N2LH5 )

For arguments sake, lets consider a motor supply of 12v, with an on load current draw of 3a at max speed / load

It seems better to use the MOSFET?

I don't think the IRF540A is a logic level device, so won't switch on fully with only 5V, so yes, maybe you are missing something.

Perhaps the wrong choice of MOSFET (IRF540A) for the question - the 95N2LH5 is a "logic level" device (threshold voltage around 2.2v), so why is not seemingly in wider use with the Arduino?

The 95H2LHS has a threshold voltage of 1V according the datasheet I've found, which is what you expect for a logic-level device. 2.2V threshold would be pushing it too fine, unless that's the maximum Vthr. That device is not in common use because its new and hard to source in TO220 I suspect - certainly Farnell only have an SMT version. There are 1000's of capable MOSFETs out there and 100's of logic level one's, people pick what is available and cheap.

Also for some purposes you'd choose a low total gate charge over low on-resistance because of the limited driving current available - switching losses can be dominant and then gate charge is more important than on-resistance.

Oh yes, its usually better to use a MOSFET, unless you need 1.2V wasted in your motor driver (!)

Thanks Mark, Good point about the availability, I have one in my box, but had not tried to source them in the UK. Just browsing Farnell and STP16NF06L seems a choice - your thoughts?

Since there are 1000's available I start from my requirements and select the cheapest that matches the requirements. There are at least a dozen things to consider from package to gate charge to voltage to inductive pulse handling, future availability - only you can know your requirements.

So first start with deciding on:

maximum on-resistance, breakdown voltage, gate drive voltage, package style.

then start hunting. The cheaper devices are usually cheaper because they are more popular which tends to imply they (or improved versions) will be around for many years or decades to come. Its not just a question of saving money! (I don't mean cheaper on eBay, I mean cheaper from an electronics distributer!)

Seems not too bad of a part:
RDS(on) Static Drain-source On

VGS = 5 V ID = 8 A, 0.08 ohm
VGS = 10 V ID = 8 A, 0.07 ohm
Power dissipated in the part at 3A,P=I^2 x R = 3 x 3 x 0.08 = 720mW, so you might want some heatsinking on that.
Compare to TIP120 with Vce sat of 2V at 3A IC, P = IV = 3A x 2V = 6W!
SO: MOSFET definitely the way to go. Lower Rds even better if you can find one, like AOI510, <4mOhm. P = 3 x 3 x 0.004 = 36mW

If there's any chance you can attach the transistor/fet to the build plate (in the case of an aluminum build plate) then all of your questions about low RDS or Vce values go away. You'd be running at "100%" efficiency even using TIP120s. 100C, the typical temperature for a build plate, is not a problem for TO220-style transistor packages.

Granted, this is not practical if you're using one of those PCB-style heater plates and you still need some kind of load like a power resistor or such.