[boxhead]

Sorry Merv, it is the single pipe look I dont like.

I can understand them being fitted if running live (easier and cheaper) but if your fitting them purely for "the look" then unless your car is a 2000 series FIA that had the small single pipe that popped up from under the sill and ran down the side, then any single pipe will not only be fake, but really look fake.

I realise everyone has different tastes, and yours is obviously one that does indeed like the look of a single pipe.
In regards to what material, well your adding weight anyway (since there fake) so a couple of Kg difference between a thin wall steel of thicker wall alloy will be marginal (I doubt you would go with thin alloy as it will dent easily)