Advantage... Motor is out of the water so no seal issues (motor gets wet, motor dies)
Impeller assembly is smaller, easier to attach the exit plumbing to.
Less expensive to buy
Disadvantage....
If the space overhead is limited, may not fit
Most have a plastic bottom assembly, not cast, could be an advantage or disadvantage depending on the amount of crud it has to pump out.
Float switch sometimes fail but are easy to remove and replace
In my case I want a thermoplastic lower assembly as I dump the acidic condensate from the condensing furnace into the sump as well.