The inon lens are designed to work at zoom of 80mm and have magnification of 0.23x so that makes 18.4mm equivalent which is 100 degrees diagonal
Amongst various benefit of an aluminum housing:
1. Depth : rated to 100 meters vs 40 or 60 of plastic
2. All internal parts can be serviced some plastic housings are not serviceable
3. less fogging. Plastic housings tend to fog on the port as this is glass and conducts heat better than plastic, aluminum housing fog less as condensation happens first on the metal
4. more robust it does not crack
5. Take heavy wet lenses
Now the price difference is quite hefty for the G series if you compare dollar prices
Canon OEM housing $349
Ikelite non TTL $550
Fantasea $575
Nauticam 1050$
The Nauticam housing is 3x the OEM housing. You would argue that as the OEM housing in effect allows you to do exactly the same things except the depth rating you should not bother and put up with the worse control (having to hit two buttons to control the dials)
Fantasea to OEM is 65% more. It is better built it gives you a moisture alarm and you can control the dials with no problems however does not take wide angle lenses. This would be a good choice if all you do is close up work
The Ikelite housing for me makes no sense and it is outperformed by the fantasea
So my summary would be:
If budget is an issue and you don't change settings a lot (two G series has two custom modes that would be sufficient for more situations) you go for the OEM housing
If you shoot only macro and close up and want to change settings a lot you go for the Fantasea
If you shoot CFWA and want a product that will last over 3 years (at this time any plastic housing is ready for the bin) then you get the Nauticam
Accept most points would add ... on your options... I don't want to use strobes or WA lenses, however I do want to be able to access all of the camera functions underwater ....
You cannot operate all of the controls on Canon housing, nor can you raise & lower flash ...
That was my main reason for opting for Fantasea
Disagree on one key point... why would polycarbonate be no good in 3 years ? .... I have used mine well past 5 years and no problem.
Only change a set of O rings once in that time.
There is no reason to expect Polycarbonate to only last 3 years.
I'm sure Ikelite, Fantasea & Canon would completely disagree with you that Polycarbonate is only good for 3 years.
I still have a polycarbonate housing for my old Canon G3 and that is
over 10 years old .... and is my loan camera to dive buddies, used, abused and still works as good as when new.
Your need to upgrade camera will come in before housing will need replacing .......... and that again is a good reason not to spend more on housing than value of camera (but that is just my personal money guideline)