It's a matter of personal preference. I much prefer a three wheel. It's more maneuverable, corners better, and has more leg room. I have ridden my friends 4 wheel on occasion, and it drives me nuts. It feels clunky, confining, and hard to steer.
My friend much prefers a 4 wheel. She says it is more stable and more predictable. She likes being able to prop her foot on a front fender to straighten her knee more, when it's giving her trouble. She has ridden my 3 wheel on occasion and it drives her nuts. She says it feels wobbly. She particularly doesn't like the tight turning radius as she has problems cornering in tight spaces, and it sometimes lets her get into situations where the back fender runs into the corner.
So it's kind of up to you and what you feel comfortable with. I believe the 4 wheel is definitely more difficult to parallel park on the bus, but that's not really much of a problem anymore. It appears that policy has changed about bus drivers parking your
ECV for you. Now they will usually offer to let you just pull onto the bus, near the spot to be parked into. Then you take your key and take a seat, while they put it into free-wheel and push it into the spot and strap it down. If they don't offer, feel free to ask.