I've been wondering about the two Ultramarine for the longest time and decided to finally compare the two. By the way, I'm currently using French Ultramarine.
Here's what Daniel Smith say about Ultramarine Blue on their website:
Ultramarine Blue plots cooler and bluer than the more saturated French Ultramarine. Temperature aside, both blues have equal permanence, lightfastness and transparency.
Ultramarine Blue is slightly less granular in concentrated washes. For less saturation, sedimentation and cost, use Ultramarine Blue straight, for vibrant crayon-like color or mixed with a cool red for dark, effective neutrals.
And this is what they say about French Ultramarine:
This medium-to-dark warm reddish-blue is highly lightfast and of medium tinting strength. Its sedimentary quality increases its versatility.
Mixed with various portions of other blues, French Ultramarine is a wonderful sky pigment. Modify it with Quinacridone Gold for delightful greens that remain color-coordinated. Mix French Ultramarine with Quinacridone Burnt Orange and be rewarded with an amazing range of blue to brown grays. Mixed with either Quinacridone Rose or Pink, a range of purples result. For another important mix, see Hooker's and Sap Green.
I'm not so sure about their "less granular" for Ultramarine Blue. I actually though that they are more granular, more textural. Anyway, the difference is small.
From the colour mixtures that I've created, I wasn't able to spot any difference as well.
Can you spot the difference?
Which one do I prefer? Well, both, since I don't really see much difference. You might want to check out Jane Blundell's comparison of the two Ultramarines and other brands as well at https://janeblundellart.blogspot.com/2013/06/watercolour-comparisons-1-u...
Here's the video of how I created those swatches above: