Fireplace Flue: best type of flashing for Colorbond roof?

We've just had the triple flue installed for our inbuilt wood heater for our new home.

We paid extra to the fireplace supplier to have a Dektite flashing around the flue where it penetrates the corrugated Colorbond roof.

However, when the installer was finished we noticed that they had installed what appeared to be an aluminium flashing around the flue (a flat piece of aluminium banged down to match the contours of the roof sheet around the edges, with silicon sealing the flue to the flashing), rather than a Dektite.

We asked the fireplace company and they checked with the installer who told them that he reckons that aluminium is better than a Dektite because the Dektite will eventually leak. They did however say that while the aluminium flashing was more expensive than a Dektite (we have no idea how true that is), they would happily get the installer to change the flashing.

We have no idea whether aluminium is better than a Dektite, or if bare aluminium in contact with Colorbond will cause a corrosion problem.

Does anyone have an opinion as to whether we should leave the aluminium flashing or get it changed for a Dektite flashing?

Thanks!