Group exercise and personal training are very different and require different levels of knowledge and certifications. You should not be personal training if you are not certified/qualified. It is very beneficial to have both certifications in my opinion, as they do overlap in some areas and have helped become stronger as both a trainer and group instructor.
There are many certifications to choose from, and I would go with an NCCA accredited one. They will all have different requirements and the amount of time necessary to prepare for the exam is all individual.