Hi David
The best thing to do is to find out what towcar you need in terms of weight, power etc and when you've narrowed your choice down to a few 'possibles' look at things like fuel economy, road tax etc.The automatic gearbox version might be in a higher road tax band.
After you have narrowed the field still further it might come down to ease of getting in and out and driving position / comfort.When you've found the car that fits the bill just choose the automatic option.
Cars fitted with automatic gearboxes make excellent towcars and my guess is that after you've driven and towed using a car with an auto box you will wonder why you never tried it before in spite of your changed circumstances.
The main thing to bear in mind is that your car must have an adequate cooling system for the automatic transmission fluid.