Andrew Burgess wrote:

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.

Andrew's said everything I would have said about the pros & cons of in-pond,
except noise. Now, I've never used an external pump - my last pond was
quite big enough not to worry about the space it took - but external pumps
are pretty well guaranteed to be noisier than submerged pumps. Sequence's
are supposed to be pretty quiet, though.

The plumbing for in-pond pumps _should_ be simpler. I let my wife talk me
into having "professionals" build my waterfall (I suspect she - rightly -
didn't trust my artistic side). I had an in-pond pump, and the morons ran
the plumbing _out_ of the pond, then up through the bottom of the head-pond
for the waterfall. It used twice as much plumbing, and always leaked a
bit, compared to running the pipe inside the liner but under the waterfall.
--
derek