Actually it's a bit of both. A slower shutter speed will theoretically produce a bigger flame, but a faster one will mean it's sharper and better defined if you catch it right. It's up to you to decide what's best.
This selection isn't bad, 6 doesn't really do much for me though, 10, I think the lead car would be better if that was the one in focus. Oh, and 8 has a JCB type thing in it, which doesn't help!