DCP is being used by default now, and TAP is in the process of being deprecated and will be removed completely from upcoming Couchbase Server versions at some point.
DCP has better restartability / checkpointing support, so that there is less reprocessing of events. Also, for users that use Sync Gateway accelerator, DCP is required to supported the sharded DCP feed where SG Accel nodes only receive DCP events for a subset of the vbuckets.
As far as performance, I don’t have any numbers to share, but the upcoming SG 1.5 release will be undergoing performance regression testing and the expectation is that overall performance will improve.