With the WSM's one of the big factors is how well you can pack the lump into the ring, if it isn't packed tightly you get a lot of airflow in the stack that can run you out of fuel faster - remember the WSM's don't seal 100% so there comes a point where even with dampers closed you can't reduce the airflow.
Briquets stack very even and burn very even, but produce a lot more ash - but like Al said the 22.5" WSM can handle a lot of ash without any problem.
Personally I use lump for any smoke under 10 hrs. or so, then if I plan an all nighter with butts or brisket then I use briquets. When using briquets when you get past the 12 hr. mark make sure you go out and gently stir them every 2 hrs or so to knock the ash off the outside. This will allow you to go 20+ hrs. at a steady 200-220° in a WSM.