With a good keyer it doesn't matter as much as it may have in the past, but in most cases green is the better choice. Which you choose depends more on where and with what content you do your keying. If you're using equal-band capture, recording and processing -- 4:4:4 for instance -- then it can be a tossup. But any encoding that limits chroma bandwidth -- 4:2:0, 4:1:1 etc, will favor green because there's more green in the highest bandwidth channel (luma consists of more than 50% green). More resolution + less noise = better keying.
I think using blue outdoors might be favored because natural light is much bluer than typical studio lighting, which reduces the noise in the blue signal. But I doubt there's a significant advantage, because noise is only one factor.