I think the important thing to remember is there is no 'right' or 'wrong', it is what best suits you and your location. There have been lots of good suggestions on this thread, but its only natural that each of us will recommend what we have found works best for ourselves. One of the posters here Malthouse has lots of experience with SIBs and RIBs, and always reckoned that he could inflate a SIB and be ready to launch quicker than many people trailer launching.
My first question would be what is readily available where you are? If you buy a Zodiac Futura mk2 second hand and decide it is not for you, how easily can you sell it again, and how much will you lose on it? If the answers are 'easily' and 'not much or nothing' then just go for it (with the usual caveats about training / safety equipment). If you get sick of deflating / inflating, then sure buy a trailer, but don't rule out the packing away approach until you have tried it.
About 5 years ago I bought an old Avon 3.1 metre SIB with floorboards from Ebay, and combined it with a new Suzuki DF9.9 (45 kg)
For the first year, I carried it in the back of my hatchback car. This worked ok, but I got sick of the smell of petrol and it took up all the space in the car.
For the second year I bought a box trailer, and carried the boat deflated and outboard in that. This worked ok as well, but I started to think it would be easier to keep the boat inflated and just put the outboard on when I got to the launch site.
For the third year I got the opportunity to buy a very heavy duty 3.5 metre SIB. So I sold the Avon on Ebay (for more than I paid for it
).The new SIB has a single solid floorboard, so packing was no longer really an option. It is also seriously heavy (I would guess much heavier than a 4.2 metre Zodiac). So it now lives on a trailer with the outboard permanently mounted.
Over the last couple of years I have gradually tweaked the trailer setup to speed the launch and recover process, with extra rollers, tie down straps all cut to length and marked up, quick release lighting board, etc.
I suspect the one thing you will struggle with more than any other is lifting the outboard in and out of the pickup. I agree with the other comments that the only way i can fit and remove my outboard is on stable ground before launch.
My tips would be:
1. Air floor is better for packing away and lightweight. Solid floor is better for performance and rougher conditions.
2. Launching wheels are essential. I would suggest:
-Unpack and inflate boat next to car
-Wheel boat to launch site
3. Get an outboard trolley.
4. A 3.5metre SIB is perfect for 2 adults, cosy for 3 and crowded for 4.