Is there a particular hierarchy or rationale for choosing a .co extension over a .io if both are available (and .com is unavailable) for a given domain? Please advise. Thanks.
Target markets. The .IO seems to be favoured by tech startups. The .CO is still a ccTLD and if the .COM is taken, it is almost certain that the .CO will lose traffic to the .COM if it has a website.
.co is cheaper, especially for the first year reg, usually only a few dollars. io can run up to $40 or more at some places. Both seem to be in the same realm of acceptability.