is FX really recommended over DX?

I do lots of Landscape, Travel and Action photography and am contemplating going for a FX format D800/D600. I would like to know if there's real difference in FX format apart from more coverage (which is a ++point for me) ?

Also any recommendations on the glass upgrade? I have the below gear as of now.

Nikon D90 - Nikor 18-105, 50mm, 55-300, MB-D80, SB-700

Thanks.
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Nikon D90 - Nikor 18-105, 50mm, 55-300, MB-D80, SB-700