The practical difference is that the 'raid1' mirror segment type always stores its logs (in fact, metadata subvolumes) on-disk on the same PVs as the lv being mirrored. You no-longer need a third pv for the log, or to store the log in-memory, and consequently the
--mirrorlog disk/core/mirrored options to
lvconvert are not applicable to the raid1 mirror segment type.
Another practical difference is that you need an additional 1 PE on both PVs (original and mirror) in order to store the log(s), allocated when the mirror is created. If you see an error like
1 extents needed, but only 0 available when trying to create a raid1 type mirror with
lvconvert, this is probably a failure to allow 1 PE additional space for the log on each of the PVs.
Since about September 2013, raid1 has been the default mirror segment type in lvm2.