whats a Sequence?

Brand of pump. I like them.

I was looking at external for ease of cleaning and mag for low power.

Mag does not imply low power. Mags advantage is no shaft seal to leak.
Since I've never had a shaft seal leak this is not a big advantage IMHO.

Whats the advantage on having it IN the pond?

Out of sight (maybe), simpler (or no) plumbing.

Disadvantages are oil leaks, shock hazard, takes up pond space.
And if your filter is in there with it, tough to clean.

HTH