From a SEO perspective the domain shouldn't really make any difference - or at least not a major one. As Google often emphasizes they try to rank by relevance for the user. There's one main exception, which would be ccTLDs (country level domains). And while .CO is a ccTLD, it's considered a generic domain by Google.
https://support.google.com/webmasters/answer/1347922?hl=en
There were some TLDs that would negatively affect ranking, but .CO wasn't one of them.
I do think you should always try to own the .COM though - as domains get passed on by many means, i.e. word of mouth etc. Users are used to seeing/hearing .COM more so than .CO, so they might just try to visit the .COM instead of going to your site.
