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Google makes semantic level query assumptions by default now- that's part of the function that allows google to suggest queries as you type.  To address your question more directly, it depends on the individual terms and whether or not Google is giving complete, partial, or no equivalence to those terms.  Non-profit and nonprofit are treated nearly the same in organic ranking- the top results are identical and the remaining results have more variance. You can experiment with this idea and you'll see some of this phenomena. As for "concrete evidence"- you aren't going to get it.  You should study the results of your individual terms' rankings because that's exactly what Google did to weight potential results... at least, it's a part of what they did.  They have a vast army of "raters" that look into the relevance of query/results and assign a value to the search engine's success/lack of success.  In the end, a load of data is collected in various categorically-structured groups of search terms. Are they using the data attached to these unique categories to craft a more relevant set of results?  I don't know if they are, but I find it hard to believe they flush all of that expensive data down the drain just to use an umbrella algorithm to spit out all query results. In the end, I think you'll find that worrying about the minor difference between the two is wasted time if you're looking for a concrete answer.  To avoid the pitfall, stick to a single convention on each individual page, but vary the convention from page to page.  When you search one or the other, watch how Google bolds the alternate terms in the list of results..  Compare the 1st page results of the two alternate terms..  are you seeing a pattern? With the other components of SEO on-track, it effectively won't matter at all.

Hi, This video from Matt Cutts talks about how Google will best handle the 301's and how long it might take. If the site is large and database driven, then using database lookups to generate your 301's would probably be the best solution. Long lists of 301's have a potential to create processing issues - not to mention the pain associated with having to create them! I would also do the work to at least attempt to get external sites to change any existing links. Given the legal ramifications, I would not want to leave any anchor text out there for the other party to take issue with if I could avoid it. (I would also keep a detailed record of all the work done in this area, just in case there is any kind of legal challenge). Hope that helps, Sha