INFRARED REMOTE CONTROLS... in conclusion, which one's more common - 940nm or 880nm?

Ah, the standards and the lack thereof!

It took a while to figure out that 38 KHz is the single most common carrier frequency for consumer electronics. What I can't seem to find is the infrared wavelength that is the most used in sensors - whatever source I come across is vague - anywhere between λ=880nm and λ=950nm

So which one is it? I need to finalize a universal remote transmitter design and I have to choose what LEDs to use. Or, does it not matter much? Are IR phototransistors "wider band" somehow? Even if they are, they must have a sweet spot but what is it for most of them?

Your input is greatly appreciated!

It doesn't matter, as long as your emitter LED's response peak matches the receiver photo-transistor's peak.

Get and read the data sheet(s).

Yeah, well, as I was saying, this will be a universal remote (or as close as possible) that will control a variety of devices already on the market. Thus it's not my call to choose the receivers; they're already out in the wild and you probably have some of them in your house.