I was on this year's Transatlantic Splendour from Barcelona to Galveston and the sailing was wonderful. We had smooth seas all the way and some days the ocean was almost glassy smooth. Hope this holds true for you.
As for formal nights, I believe there were 3 formal, about 3 or 4 casual, and the rest were all smart casual. On formal nights there was a split between tuxedos and suits with also many in just sport jackets and ties and some with jackets and no ties. I would say you would be totally comfortable going either route. I did notice that each formal night became progressively less formal with the last one having those wearing tuxes in a true minority.
Enjoy the cruise. All the sea days are incredibly relaxed and enjoyable. In fact I am also contemplating this cruise as well since this year's was so pleasant.