We have both a thetford flushing portaloo and a kampa khazi!
The kampa khazi we use for shorter trips, wee's only and wash it out each morning. It is a bit smaller than the Thetford portaloo. The thetford is lovely because you can flush it, we will be taking this when we are away for 2 weeks in the summer.
The kampa khazi is much cheaper than the Thetford portaloo's. I hate having to look at the wee in the bottom of the kampa khazi but it is easier to pack as well as being very cheap!