Well I just learnt that aluminum does not rust, it only corrodes, which explains why so many options are aluminum.
i am no expert but is aluminium not used as it disperses heat soak more quickly?
Many of the companies here in the U.S. offer the pipe only in aluminum (CPE, Mishimoto, MBRP). I am not sure about how aluminum would fare against stainless steel in the heat soak department (I would guess they would be close?), but I know stainless steel runs a bit hotter than plastic (MAP provided this in their R&D thread, they had slightly higher charge temps but better overall performance). I would assume that aluminum would run a bit hotter than plastic as well, although MBRP does claim "Lower Temps" on DHM's website, however I haven't seen any data to support that so logic is telling me that plastic would still run cooler than aluminum.
There is probably someone more knowledgeable on this subject than I but I think replacing the OEM hot-side pipe would result in negligible differences in charge temps as it is before the intercoooler, I think replacing the cold-side OEM plastic with aluminum/stainless steel is where you would see the differences in charge temps.