I probably like resin the best. Reasonable quality and accuracy, adequate heft, not too hard to carry around if you travel with it. Usually, not too many parts, especially if made by a gaming company.
I find that I don’t like the light weight of plastic. Smaller vehicles have tended to slide down slopes more readily ( though this will vary with the type of terrain that you use) and they will “float” above the ground if they hang up on the slightest bit of lichen. Additionally, I’ve had more damage occur in transit with my small number of 1/72, 20mm plastic models, than my hundreds of 15mm models over the years. They do offer a light load, and great detail, if lots of pieces for assembly.
I only have one metal vehicle in that scale. I like the heft, but even a small army of them would be a load to move around. It is an older model with overly thick tracks and fenders, which may be common based on sculpting methods and casting/flowability considerations.
I don’t tend mix the same type of vehicle from different manufacturers, as scale and detail can vary so much from one manufacturer to the next, though with some, it doesn’t vary that much.