Different types of conference publications
For the value of the publication, it will matter more how it is published than whether it's a workshop or conference. If the proceedings are published by a well-known publisher (i.e. with ISBN/ISSN), a workshop paper might be as "valuable" as a conference paper. If on the other hand there are no formal proceedings or just something like a folder, it would be less "valuable". That usually leaves the opportunity to publish the same paper elsewhere though.

Most workshops in CS don't have formal proceedings, so in general it would be better to publish at a conference. That said, in some cases the reviewing process for a workshop/symposium might be more stringent. As smaller events are more specialised, the feedback you get on your work might be better than at a bigger event.

I personally would always go for publication at a conference if I thought the content of the paper to be good enough.