Long : more stable laterally - more resistant to snaking.
Having the rear wheels close to the tow hitch helps as well.
Generally LR 110 is considered better for towing than LR 90.

If you REALLY want to know, then look at:

Authors: Fratila D and Darling J
"The simulation of coupled car and caravan handling
behaviour", Vehicle System Dynamics, number 26, volume 6,
1996, pages 397-429, ISSN: 0042-3114
Ref ID: 1034, Ref Type: Journal, RAE: 2000, Academic Year: 1996

and

Authors: Darling J and Standen PM
"A study of caravan unsteady aerodynamics", Proceedings
IMechE Part D Journal of Automobile Engineering, number 217,
2003, pages 551-560

Abstract: The high-speed stability and handling
characteristics of car-trailer combinations are affected by
both road and aerodynamic forces. While the tyre to road
interaction is well understood the action of gusts, passing
large vehicles and even small steering inputs will disturb
the symmetry of flow and generate aerodynamic forces and
moments which are sufficient to affect the handling of the
system. Although accidents caused by high-speed instability
are relatively uncommon a better understanding of these
aerodynamic effects will improve safety.

In this paper a series of wind tunnel investigations using
scale models are presented. Steady state investigations were
used to measure the aerodynamic properties of the
car/caravan pair while a novel technique was developed to
measure the aerodynamic damping derivatives in yaw and side
force for a caravan model. These damping derivatives were
shown to be destabilising in most cases of sideslip and
stabilising in yaw although it was demonstrated that high
damping derivatives were attained at certain frequencies of
excitation.
Ref ID: 2392, Ref Type: Journal, RAE: 2004, Academic Year: 2003

!!

Tim A
Design Engineer, Bath.

MJ came up with the following;:

LWB

A lot of our members (http://www.pocuk.com/forums/) tow with SWB, even large
caravans.

Frank