Since one of your chosen themes is the isometric one, I'd stick with the new isometric buildings to reinforce that part of it. They look pretty cool.

Even though this is a 2 month 'Diamond special', you will find time slipping past quite quickly, so even if you aren't completely satisfied with the ocean part of it, why not get a head start on the forests and buildings and revisit the ocean later? Having it 3/4 done half way through is a huge boost - as opposed to only having done the ocean at the half way mark.