autos give the horses a smoother ride
also - if you prang yourself at a show and feel awful - you can still drive them more easily with a sore arm or leg than you can a manual
spares aren't a problem as the main difference is usually only the gearbox and those with autos either work - or they don't - and when they don't it's probably going to cost more than the price of the lorry to fix the g'box.
no clutch though to go wrong and auto boxes are ususally reliable until they break completely whereas manual boxes have the foibles which can be fixed but lead to ongoing costs
transits though - if it's the sort with say and Ifor williams container on - are known to sway and give horses a bad ride if it's one of the old tranny conversions
so I'd avoid if it's one of those - manual or auto