If you want something that works well out of the box, I think bootstrap is a better solution. Many rails developers just use the plain bootstrap for their apps. Also bootstrap seems to have a larger installed base and a larger community surrounding it.
If you want to get further into the design, take a mobile first approach or prefer scss/sass over less, or want to highly optimize your front end assets especially for mobile (zepto vs jquery, interchange responsive images, etc.) Foundation would be a better solution imo.
Both provide a very simular set of functionality, although there are some slight differences.
Both provide a somewhat opinionated design out of the box. A lot of people say sites look bootstapy or foundationy. This is usually because there is very littile customization done.
If one is more what you are going for, that could be the ideal solution. If you go to the main site for each, you will get a pretty good idea of what that design style is.