> LED output is spec'ed in mcd; IRED output is spec'ed in mW/sr. (I think.)

Millicandelas (mcd) and mW/sr are not similar but not easily converted.
A candela is derived by taking the luminous output in watts per steradian
and adjusting it for wavelength based on the sensitivity curve of the human
eye.  Of course, some eyes are more "human" than others.

> I am using a phototransistor as a detector. From it's point of view, it
> won't care if the radiation is visible or invisible; it can detect either.

Ah, but it WILL care.  It has a sensitivity curve just the same as a light
emitting semiconductor has an emission curve.  Pick an emitting LED
at or nearer to its peak sensitivity and you'll have a good deal more luck.

> Red LEDs would be acceptable in my application, but the invisibility of
> IREDs would be preferred, if comparable cost and radiant output can be
> obtained.

Experimentation may be your best bet, although generally speaking,
using IR for signaling to any semiconductor light detector is preferable.

Maybe if you told us more about the application...

Mike H.
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