^^ In general : with increased series number the backup time and ability to run more power consuming devices from the UPS will be possible.
Now consider the load situation with an example : with a 600VA ups you can put a max load of 360W and with a 800VA ups you can run a more power hungry rig .
Now coming to the backup time : if you draw 360 out from a 600VA UPS constantly the UPS will only give backup time of of around 2+ mins
say you have a rig which consumes 200W so a 600VA UPS will provide around 8-10 mins backup time and with a 800VA ups will get better backup time
So in short while shopping for a UPS you have to consider two things : how much load you will put on the UPS and how much longer you need backup time from a UPS.