Welcome to a fantastic site!
Whereabouts are you? Port Refrew? Tofino/Uculet? Gold River?
I'd recommend getting in touch with Rob or Trish from Hi-Tech Inflatables in Parkesville,
or Anthony at Orca inflatables in Vancouver
or, take a trip over to sunny, warm Manitoba (the ice should be off the rivers and lakes in the next few weeks), and I'd be happy to take you out in my hypalon Dive 470.
Rob at Hi-Tech has a demo model PVC Dive 470 with double skinning for extra abrasion resistance. He might also have a surf 420 on hand as well.
The PVC boats tend to be a bit more rigid because of the material is less elastic and that they can be pumped to a higher pressure. For a boat designed to be as high performance/and light as possible such as a zapcat, PVC would have clear performance advantage over the same boat in hypalon. For a heavier, more utilitarian boat such as the Dive 470, the performance differences between PVC and Hypalon would be much less noticeable - especially if the PVC is double skinned.
Hypalon will have far greater resistance to UV degradation than PVC (although both materials should have a tarp cover when not in use). Hypalon will also remain more supple and will not fatigue harden over time like PVC will. Hypalon will also maintain that suppleness in the cold and is also less affected in very hot conditions. For me, this is very important because in Manitoba the temperatures have an 80 C range from winter lows to summer highs. I've used the boat in Mid-late November here, where on the faster flowing rivers, the water might still be open at -20C. A PVC boat would almost certainly crack at those temps. This is probably a non-issue on Vancouver Island or the mainland coast.
Gemini hand glues the seams of these boats regardless of whether it is hypalon of PVC, which is a good thing because it makes them repairable.
I have not had personal experience with the surf GRX 420, but from what I understand they have almost zapcat-like performance with the practicality of a round nose. 'Ed the Duck' on the board here has one in hypalon, and I believe there are a couple of guys on here with PVC versions as well. 'Easy Rider' has a Ceasar Surfcat which is very similar to a PVC Surf GRX 42O. Not to put words in their mouths, hopefully they'll chime in, but I know that they are extremely impressed with the performance and quality of these boats.
I struggled a bit deciding between the GRX 420 and the Dive 470, but decided on the extra space and weight capacity (bigger tubes) and general bombproofness of the 470, at the expense of the extra performance of the lighter GRX 420. I also went with floorboards that were thicker (3/4") than standard (5/8") for additional rigidity with heavy loads.
As for power requirements, I have an older 2 stroke Yamaha 40 which will give me close to 40 km/h with me and another couple hundred pounds of gear & fuel in the boat. I would expect that one of those new 30 hp Etecs would perform pretty close to my older 40. If you were to put that on a GRX 420 (which would weigh about 150 lbs lesse than my boat) I'm sure that you'd be able to go considerably faster your average passenger would like.
As for seating, there are all sorts of things that can be done with some fabrication. I much prefer that my passengers sit on the sponsons while I'm piloting the boat so I can see where I'm going (I sit forward facing on a seat while tillering with my left hand). With the 22" diameter sponsons, the sitting height for the passengers is very comfortable, and they are well 'inside' the boat.