I ahve experience of Plas Menai, all be it a few years ago, indeed I taight a Level II there as a follow up to my Instructor Qualification 6 years ago.
The TIDAL waters of Menai will put you in good stead for future activity, especially if there is a Spring Tide running.
As the National watersports Centre for Wales their staff should be up to scratch, yes the office staff can be a bit vague but the on water staff have always been very good.
If you still want to have a look at othercenters, all well and good, I am sure that they offer good teaching, I note and can agree with some of the comments about dinghy/powerboat instructors, but thier overall awareness should be an advantage....note should.
Whereever you go, be sure to buy a copy of Peter White's excellent book called Powerboating.
It is due to be updated due to the emergency stop being taken out of the syllabus, and a few other matters.
What ever you do, make sure you feed back to the school of your choice how you felt they handled you.
I always seek feedback.......AFTER handing out the certificates if deserved.
Have fun, get wet and stay dry!
Don't go looking for bad weather, it will find you soon enough!