To Glue or Not To Glue, that is the question?

Glue baby, glue.

I would recommend gluing all the slats with a water proof glue.

Alrighty looks like Gorilla glue is on my buy list.  

Thanks

I think gorilla glue, the polyurethane species, it not well suited for outdoor use.  I believe Titebond III would be a better choice

Seriously?  Polyurethane glues, or whatever gorilla is are similar to time tested outdoor construction adhesives.  I don't generally use Gorrilla glue, and never use Titebond III, so I am not pushing it, but for specific reasons I used it to rebuilt the rear deck on my trimaran.  Generally gluing wood that will just sit outdoors doesn't work very well unless the whole project has engineered encapsulation.  Or some really appropriate wood like teak, but even that isn't so great judging by the many benches I have examined in the UK.

Glue and pin.

Forget the Go Rilla.  It will fail with the greater outdoor wood movement. Especially if your m&t's are not very tight fits.

Glue? Epoxy.  Pins? 5/16"  hardwood dowels, through-drilled and sanded flush.