If size is not an issue spanky then get a normal ATX. It will be cheaper and will allow a lot more expansion. At the end of the day the choice is yours. The ITX will be small and discreet. The ATX will have better cooling space and expansion possibilities.
You need to decide where you want to put the pc. Can you afford to lose space in the boot or is it better under a seat or in the glove box. Once you've decided that then can you decide what size mobo you require