I don't see the double axle caravan necessarily gives more room, nor up to quite high weight levels, greater weight capability.
The latter might have been the case back when SA chassis could not carry the mass they can today, however even when we last purchased in 2007 our SA had a 1900kg Alko light weight series chassis.
Not long after that Alko upped the light weight series to 2000Kgs and I believe with the light series they have gone higher, 2100kgs?
Caravans up to the 2000kg will cover I suggest most of the range we as UK vanners purchase for other reasons? So, in fact most of us don't actually need a double axle to carry the vans of the weight we choose to own. The picture changes where very large vans are concern, there being over the weight limit of SA caravan chassis, there is no light chassis alternative.
The double axle in like for like cases lose more internal space because of the greater length of wheel arch intrusion. Along with this comes the greater compromise on layouts because of these longer wheel arches within the van.
The double axle does tow quite a bit less twitchily in yaw simply because the damping from the tandem tyres inherent resistance to track other than in a straight line.
However, this less twitchy state should not be confused with critical stability, as once the energy builds up a twin axle can certainly go seriously unstable. The SA in this respect is different in that it tends to remind you its getting unhappy before it literally "throws a wobbly".
Choice might also be effected by the intended type of campsites and camping used. On hard surfaced sites and accesses the twin raises no issues, but go off the hard stuff onto grass the inherent resistance to turning results in the ground being torn as the easiest relief of the tyre windup. From that the rallying and 5 van site type caravanners "tend" to keep to single axle vans, at least if they care for the interest of site owner's and follow on users.
This tyre resistance I have found can bring a plus in it provides a bit of reticence to turn that gives reversing a "softer" manner. This can make things easier, at least for those of us not masters at reversing, even if probably making positional precision less easy to achieve by those that are.
Other pluses for twins, the wheels are way lighter to manhandle, and come a full on puncture you might be able to coast to the nearest layby .They seem to run lower tyre pressures so each individual tyre is more compliant which has to yield a softer ride over harsh surfaces.
For our own needs I would now only buy a SA, but that is because of our particular needs, our camping and our maneuvering challenges at home.