Theoretically If site A is called: buicksforsale.com and sells buicks and Car parts and has been around for 5 years with a PR3 and a couple hundred links but you are re-doing the site to market it and optimize it for Ford motor cars for sale and car parts and the new domain purchased and new company name is BFordCarsForsale.com should I: 1) Use a 301 Re-direct for all of the old pages on the site and if I do that will it eventually pass along page rank and link value to the new domain? 2) Re-direct the Bfordcarsforsale.com website to the original website since its being redesinged for Ford cars anyway 3) Make 2 seperate sites?
If you are making a new brand, I would choose the redirect option. You might not get the same traffic as now, but close enough.
As your site buicksforsale.com if 5 years old as you say then it must have good SERP impression in Google. Doing 301 can pass your link juice to your new domain but you may not retain those keyword position as the relevancy may dilute. So i will suggest to use two different sites. But if you don' want to retain the previous rankings then you can try for 301. But don't use other unethical redirect.
i choose option 3. buicksforsale.com for main domain and BFordCarsForsale.com for support main domain.