Depends how sensitive you are. I still have an Elsan Bristol "bucket & chuck-it" but you need to be SAS standard living-in-the-wild to stomach one of those. It's not too bad using it to crap in, but speaking as an ardent admirer of the British beer retail industry (a.k.a. pubs) I often have to throw up in the damn thing.
With 15 people scheduled to use it, I think the first thing to consider is "who is going to empty it?".
The holding tanks on the various models of Porta-Potti have differing capacity; go for the biggest!
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets