I have both MX5 Parts exhausts on my Mk2.5’s. I started off with the single exit which is a nice sound, but can be a bit droney for the first few thousand miles until it wears in. I then bought the twin exit exhaust for my other 5. I would say there is hardly a noticeable difference between them apart from looks.
I see your local to me and your welcome to come over and have a listen to each system before making your choice. What i would say is they both are well made and fitted well on my cars. They are not difficult to change and only took about 1/2 hour to an hour to change the back box.
One car has the single exit back box only and the other has the complete stainless steel centre and back sections.