As important as choosing which caravan is for you is choosing which car is best for your caravan. And the decision can't be rushed.
Several factors are involved in choosing a good towcar - engine, fuel, transmission and suspension being just four. We'll start with the car's pulling power and performance.
With a good towcar, it's not a matter of power for off-the-line acceleration, but rather out-and-out 'slogging' from a flat torque curve which peaks as early in the rev range as possible. Look for an engine with a good level of torque over a wide band of its rev range. This indicates smooth pulling from rest, comfortable performance when cruising and a good overall economy. Suitable engines can be found in medium size cars upwards, and from around 1.8 litre capacity. Most commonly it is known that the bigger the engine, the greater the torque.
Diesel vs Petrol
Diesel provides better economy than petrol, given equivalent engine sizes. However against the 'straight' diesel engine is its lackluster power, although now the arrival of the turbo-diesel has put this right. TD engines perform like petrol engines, but the full use of power will adversely affect fuel consumption. Common to both diesel and TD engines is their flat torque curves, which usually peak at between 2000 and 2500rpm. Clearly, a diesel car, such as Citroen's Xantia HDi, provides top pulling power at relatively low engine revs - ideal for towing. A petrol-engine towcar of between two and 2.5 litres, chosen for power and torque, will provide good restarts from rest, as well as hill-climbing and comfortable cruising. The TD is equal to this, but low-down torque gives it the edge on traction.
Manual vs Automatic Gears
It's probably down to your own personal choice.
The advantage of towing with an automatic is smoother restarts from rest as power is fed evenly to the driving wheels, reducing wheel-spin. If a sudden burst of acceleration is required, a 'kick-down' on the accelerator will achieve this. The main advantage of a manual gearbox is its ability to match engine speed to optimum torque. An automatic's fluid drive allows a certain amount of 'slippage' between the transmission and the engine, resulting in some loss of power and a consequent increase in fuel consumption.
Auto gearboxes can also suffer from overheating when towing, especially in hot climates or on extended gradients. Some car makers exclude certain auto transmission models from towing, or require an additional transmission oil cooler. Check with the dealer or manufacturer. The car's handbook will tell you what is required.
Obviously, the downside of a manual gearbox is simply the mechanical need to depress the clutch and change gear - no problem on clear roads, but it can become a chore in town or on congested roads.