One domain would be better in your case since all of the content is around roulette. Presumably, the reason you would want separate sites is to rank for a variety of different keywords (ie: site A is keyword 1, site B is keyword 2, etc). However, you are better off building one site about roulette, building the brand and expanding the keywords for the one domain. This also assumes that you are planning on using ethical “white hat” tactics to gain rankings.
Sub-folders should be used over sub-domains. Google considers a sub-domain a totally different website in most cases and doesn’t transfer the strength of the site to your sub-domain, so essentially it is like starting with a brand new domain altogether.
A 9 year old domain is fabulous, especially if you have it targeted to online casino traffic. You should have a much better luck marketing that old of a domain as opposed to a brand new domain.