I don't think too much has changed between the two in the last few versions, except if you are interested in single sourcing. (Frame has better DITA support than earlier versions.)
I find Frame's two greatest advantages to be automatic error checking when updating books and single sourcing. As far as whether Frame is worth the investment, in my opinion it really is largely depending on whether you are planning on using any of the single sourcing features: conditional text, shared chapters and shared sections (text insets). After using Frame for a few years, I've come to really like it. I can't imagine not using some level of single sourcing, and the error checking when updating books does reduce some of the stress when finalizing a book. Regards, Shmuel Wolfson Technical Writer 052-763-7133