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image type doesn't really matter at that size, and with what you're using the file for.
I like my HQ scans to be at least 3000px high, which, for tank raws, can only be achieved with 300dpi + 140-150% page size. Or with 600dpi (which takes way longer to scan).
And I agree with nehaali - at this size, file type doesn't really matter. PNG and TIFF take are quite heavy, so they're not for everyone. Maximum quality JPEG should suffice.