Based on our record, Lightning Design System seems to be more popular. It has been mentiond 2 times since March 2021. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
LWC is supposed to be their approach to play nice with web standards and appeal to a broader group, and "web components" without the lightning niche specific stuff can get you close especially if you use https://lightningdesignsystem.com for your off-platform-hosted page and web components. Source: about 1 year ago
This is a bit more of a broad topic than your original question but the lightningdesignsystem.com website is always a good resource for various specifics on styling and SLDS (aura). Source: over 2 years ago
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