How long it takes to get out of the parking lot depends on a few things, such as where you choose to watch Illuminations, the time of year you are traveling, whether there is EMH that night, and where you end up in the parking lot. It usually only takes us a few minutes to leave the lot, but there was one time that someone had an accident which blocked the parking lot exit, and we sat for three hours without moving (and of course, the buses couldn't leave either). They run the trams as quickly as possible when the park closes, so we've never had to wait too long for one of those either.
If you are traveling during one of the slower periods of the year, and are watching Illuminations from the Future World side of the park, you should have an easier time whether you choose the bus or to drive. The WL bus stop is one of the closest bus stops when you exit the park, which helps a lot. If you prefer watching Illuminations from the far side of the lagoon, I find it's better to make a leisurely exit and let the crowds get on their way (and out of yours). If it's an EMH night, and you choose to go back to the resort right after Illuminations, I would recommend the bus over driving, as most day guests will be leaving the lot, but many resort guests won't be heading to the buses, so the lines are very manageable.
If I'm traveling with my Mom (who can't balance standing on a bus), I usually drive to the parks (except MK) and if I'm with friends or on a solo trip, I usually use Disney transport.
