Put the kettle on and I'll tell you - no, on second thoughts, don't do that unless you're plugged into one of the higher-rated hookups!
Because it's all about the total current drawn by the appliances you're using. If your hookup is mainly for keeping your leisure battery charged, and maybe running the fridge to save a bit of gas, then the lower rate ones will be fine, but TVs etc draw more, and as soon as you start heating anything (water, people, etc) you're really into the heavy duty stuff.
So depending how much electricity you want to use, you may or may not need the higher rated pitches - which will probably cost a bit more than the lower ones where a choice is offered on the same site.
No doubt someone will be along in a minute to give you chapter and verse on how many amps you need for different appliances, but I hope this gives you a general idea of what's going on.