DT starts do a sampling of monitoring transactions when the application is under heavy load. It does this in order to keep performance overhead low.
AppD monitors 100% of transactions 100% of the time for a max of 4% of overhead.
DT is unable to show transaction lock on DB. It does show "wait time" but you need to have a separate DB diagnostic tool to see a transaction lock.
AppD will catch a DB transaction lock out of the box.
