The 4-wheel has a higher weight limit, which is a really great benefit- esp for Disney when many older kids will still want to be in a stroller (my 6yo wanted to sit in our city mini double much of the trip...he squished himself in).
I have a city mini double - no double will ever turn as easy as a single, but the double is a GREAT double (I have owned many doubles and it's my favorite side by side double of them all).
I plan on renting a 4-wheel for my older son on the next trip (I have twins + older son) b/c with the double + ride on board this past year it was too heavy for me to push with all 3 on it.
If you are buying- i'd get a 4wheel again so you can use longer (heavier kid)... but if renting and have a smaller kid i'd get the 3-wheel because the 4-wheel weighs a little bit more.