I think I heard some negative feedback on external mounting, but it was related to heat. I can't find the post....
Ecotech claims either is fine.
In case nobody else chimes in with first hand knowledge...
It's logical to assume it's "louder" when plumbed external because water is a pretty good insulator. I doubt it would be loud enough to be noteworthy though.
I have 2 Vectra pumps.
The L1 is my closed loop pump and the M1 is my return pump.
I replaced a Reeflo Dart/Snapper with the L1 and was shocked at how much quieter the system was.
So much so, that I could now hear the 1 year old Eheim 1262, which I always assumed was as quiet as any.
So I bought the M1 to use as my return pump, just because of the silent operation. I don't consider the cost a waste ( as some would). I put a premium on noise level.