I would always prefer the brand name to keywords.
It looks much more professional! If you see a website at companyname.com you know it's a website for that company. If you see a website at cheap-bits-and-bobs.com, you know it's a website selling bits and bobs, supposedly cheaply, but you don't know who is behind it, and it looks seriously dodgy. After all, everyone knows that to buy books, you do to amazon.com ... but what has a rainforest and river in South America got to do with buying literature? Nothing that I'm aware of! But try the keyword-laden bookshop.com or bookstore.com and all you'll find is cybersquatters who have parked the domain name. Alternatively the shorter books.com redirects you to Barnes & Noble's website, but under the name barnesandnoble.com, the company name and not the keyword.
If your content remains the same and you put the right redirects on so that every URL that used to work still works and goes to the right page, you shouldn't see any negative impact on your rankings.