I would say it also depends on your line speed, for instance having a 2mb ADSL line and a massive capped data package like 500GB a month, you will never be able to use it affectively.
Remember capped has unshaped, so it depends on what you do with the internet you use too.
Personally i found my sweet spot of 100GB on a 2mb line
Also, if you currently have an account, check your data usages and work out what you average for say the past 3-6 months.
If its less than 100GB, rather go capped.