Thanks guys for the replies.
I'm edging towards a 3 wheeler, as there's a pink one for hire!
Cool! The pink ones are very pretty!
Basically it breaks down like this -
Three wheel
ECVs are often able to turn a tighter circle, because of how they are designed. This is great because you can maneuver through tighter spaces and if you have to do a u-turn, it takes less space. Some people feel like they tip over easier, however I have never had that problem, and in fact my personal mobility device is a three wheeler. A lot of driving a three wheeler is just being aware of your center of gravity (however, as you will see, that's true of a four wheeler as well) Just don't drive like a crazy person and you'll be fine.
Four wheel ECVs are often slightly larger, and sometimes have a flat floor to put your feet on. But because they have four wheels (like a car) they don't have the best manueverabilty; for example, they need a lot more space for u-turns, and unless you shift your center of gravity, going around a corner too fast can result in you being on two wheels - and moments later, the pavement. But, if you don't drive like a crazy person... you'll be fine.
In other words... there really isn't much difference; I have driven both extensively, and as long as you use common sense and don't act like a fool, you'll be fine.
One note - make sure that you don't get the wheels into the tracks on Main Street in MK; always try to cross those at a 90 degree angle if possible. I have seen people get their wheels stuck in those tracks, and then wind up on the pavement, so cross the tracks carefully!
Have fun driving your pink
scooter!