They are quite different products. UTX modifies the landclasses in FSX, making the layout of the terrain (cities, rivers, roads, hills) much more realistic.
GEX replaces the actual terrain textures.
I use both together and they work very well in combination. If I'd have to choose only one, I'd go for UTX since it's much more useful for actual flying and navigation than GEX, which is mere eye candy. Getting both is the best solution though.