Flexjet will definately have a top of the line starting pay; however, you will not top out anywhere near what you would make at a 121 legacy or 121 cargo operation later on in your career. Plus, be ready to toss bags and jump at the business man's snap of the fingers at a corporate/ fractional gig.
Many people seem to enjoy the fractionals, though I have met people that do not enjoy it, especially those pilot's that have a family at home. Just do your research and make the best decision for you. Good luck.