I really like my 500i. Enough so that I just spent close to $400 replacing the handle and tank assembly after a little tree incident. It has become my go-to saw with my smaller Stihls left sitting in the truck. That said, a 500i now costs $500 more than when I bought mine - almost $1700 locally; I am not sure I would pay that amount when I have plenty of other capable saws.
To the OP's question, I will join the chorus of those questioning whether a 500i is really the right saw for someone who needs a bigger saw than a 7900. In my opinion, a 500i shines in that it can do 80cc work in a package that also excels at 60cc work. Though I have never run a 7900, I doubt it has any significant edge over a 7900 for work requiring more than a 7900 will deliver. I am told that a 7900 has a slight performance edge on the old McCulloch 82cc ten series saws - I own a bunch of those and have run them extensively. When it comes to large wood, my old 82cc MACs are on par with my 500i so I would dare say that a 7900 would be too.
Ron