They are completely different synths, aimed at completely different goals, and they're both great.
If you click with Diva, stick with it. Master it, and you can play a lot of classic electronic/rock/prog music.
Pigments is more versatile, you can make both classic and modern sounds with it. Although I think Diva is great, and one of the best sounding virtual synths, if I had to pick one, I'd go with Pigments, because Diva is locked to Virtual Analog sounds, while Pigments goes way beyond that.
But really, pick the one you like to mess around with the most, and learn everything you can. Down the road, if you feel you need more, then get more.