I suggest seeing the following "Technical Certification Roadmap"
The answer to your questions are enough to fill a book. The two (infosec and forensics) are like apples and oranges. Both fruits, each different. In forensics, depending on what you are doing, there could be life and liberty at hand if you are performing forensics with the ultimate route being, a court case. In infosec, same applies (slightly): life, privacy. (If you're working at say a SCADA based environment, one mishap could endanger life, if you're working in the medical arena, privacy comes first).
Aside from these two huge gaps, it works best to get a feel for both as they complement one another however, they are both extremely different. The companies hiring for these roles also hugely differ. For example, many huge law firms hire forensics experts for the sake of having talent on-hand to analyze cases, but have no true blue infosec program. Some Fortune N companies hire strong infosec teams and lack/substitute forensics with "incident responders." In forensics, chain of command, evidence, spoilation, etc., come first. In infosec, this is barely an afterthought.
Personally, I feel its best to understand both realms although they in themselves are VERY BROAD fields. Hence me posting the roadmap. The roadmap linked, shows the variations of "infosec" as there are many: application security, network security, infrastructure security, web security, SCADA based security. Same applies for forensics: E-Discovery, Incident Response, Privacy, Legal based (going to a trial).