If its not critical to the structural integrity, I'd agree with Panky as the thinner steel is easier to work. I had to use 2mm steel for the repair of the sills (the same thickness as original metal) on my car and it was very difficult to shape. I also had trouble finding small quantities of the thicker stuff, acquiring offcuts from an agricultural engineer, whereas small quantities of 0.8 - 1mm should be available from your local motor factor. The alternative round here is an 8 x 4 sheet from the steel stockholder.
Where do others buy their steel?
'It's not a proper slider'