I have their standard version without doors & it costs £12.99 perfectly good.
It does depend on the size of your cat though
Mine are bengals.
I prefer not to have a flap or take it off as I think the smells get trapped inside & it would be pretty unpleasant for the cat trying to go.....
The fact that the smells are released without a door alert you that you need to empty the tray too!
Good luck with whatever one you choose!