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!
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.