LED or LM336?

Well, the lm336 looks impressive. What the tl431 has in versatility, the '336 makes up for in thermal stability. If a reference voltage in the range of 1.2 to 1.7 best suits your needs, I'd recommend an LED. Their junction voltage changes from 50 to 100 millivolts per decade of current change, and they have a greater tempco than a silicon junction,but these characteristics could be used to your advantage.

The LED has a few items to recommend it. They're cheap, serve as a status indicator, and have only one junction to contribute noise. I might justify in favor of an LED even at the price of two or three passive support components if the board space were not critical.

If a 2.5V reference would work better, consider a few points: Is 10mA enough? Is thermal ultra-stability necessary? Will this and other projects benefit from an adjustible reference greater than 2.5V? These may lead you to try a tl431, which is available in 2% and better but lacks the zeroable tempco, and also has a different pinout.

One last thing; I didn't read all the way through the '336 data sheet but the '431 has some conditions under which it _may_ oscillate. Keep this shortcoming in mind and see if the '336 has it too.