Why should I use one over the other?
There are several reasons - first and foremost our question and user base far exceed that of Ubuntu Ask already. Secondly Shapado is a SAAS "rip" of the StackExchange software. The site software here is fully supported by the StackOverflow, LLC team and therefore receives regular updates and features pushes as well as support for bugs within the platform.
Outside of a larger/more active user base and question poll there is nothing stopping you from using both. However giving the publicity and current trends this StackExchange site will likely succeed in it's ventures whereas the Ubuntu Ask site appears to be dwindling.
96% Answered Questions
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I was unable to find the accept rate for the shapado site.