Guy makes some very good and fair points.
A couple of additional thoughts I'd like to add.
In my time working in the Lembeh Straits I did see people have more problems with polycarbonate housings than I ever did with aluminium ones. These largely resulted from slight variations in the position of the camera in the housing when increased water pressure was applied - and unwanted pressures then being applied to camera controls preventing the camera from working correctly. Of course this can happen too on aluminium housings but, as I say, I just saw this much more frequently on polycarbonate ones.
And then there is the pleasure v regrets issue. Buying a housing system is horribly expensive whether polycarbonate or aluminium. I always think it's important that at the end of the deliberations, the agonies, the unpacking and the setup that you feel REALLY pleased with what you have - and not, at the back of your mind, maybe regretting not getting that pricer (but maybe attainable) one. The luxury one as Guy describes it.
So, Guy and others make a good point: do all you can to go and have a play with both. You'll soon tell whether you are happy with polycarbonate with all the advantages and disadvantages that we have all described; or whether you are always going to hanker after the luxury option and feel. VW v MBZ (thanks, okuma!) Enjoy what you have and take pleasure in using it..... No regrets.