Depends how old the lorry is!! some of the old ones were truly dire for anything but urban work, for instance I went to look at a old layland daf early 90s had been ex council and was converted to a horse box fine untill you got on the open road and then it was so low geared it was reving its t s off at 45mph !!!
However if you're talking about the current generation of trucks they are fantastic, I love the Volvo ishift and the little 7.5 tonners like the MAN tgls and daf lf are realy good I think most trucks will be auto soon for fuel eficency and emissions, seems 50% +of sales at the moment and even more for tractor units , so yes good better economy and easy drive and beter ride for the horses ...
My view is that anything that is bigger than 2.0l (cars, vans, trucks) should have an automatic.
Saying that, an auto box older than about 10 years will be a nightmare, but modern boxes are more economical and so much easier than their manual counterparts.
And plus, if you want your horses to have a nice smooth ride, then you've got to go for an auto, just sit back, point and shoot and enjoy the ride.