Mono!! I had a stereo Rode NT4 that captured a great XY space. Unfortunately, when I wanted to use the samples, I often ended up using only one of the mics. Additionally, because I was capturing with stereo in mind, each mono mic wasn't capturing what I eventually wanted to use.
Mono all the way. Give the mixer room to mix.
Originally Posted by MichaelPatrick
I think listener interest and practical usability are both dramatically better with stereo recordings (vs monaural).
If I were designing a sound effects library today, I'd capture with some sort of soundfield technique so my tracks would be easy to mix in multichannel formats.