I've spent most of the day pursuing this, as I got both in the Strings Bundle sale.
The BEST way to learn the differences are to compare their spec pages, and then check these 2 links:
Comparing both libraries: https://www.vsl.co.at/en/Fancyboxes/Strings_Comparison
Seeing what's been moved if you choose to go the Optimized Content route on String I to save disk space:
https://www.vsl.info/en/instruments/synchron/strings-i#comparison-sy-i-pro
In general:
Strings Pro utilizes an improved process. Think Synchron 2.0. So, things are reorganized, expanded, simplified, more efficient, and in many cases sound better.
Strings Pro adds features such as: Agile legatos, Spiccatos, Detaches, Portamento, and Ponticello. The Sustains and Legatos benefit from the Cantabile improvement offered as an update to Strings I.
What's missing from Pro (still included in the Optimized version of Strings I) are Marcatos (!) Slurred Legatos (which seem to be a nice option when portamento is too much), and additional long and legato variants.
The Full version of Strings I includes shorts with more velocity layers (8-9, vs. 5), NKS compatibility, and 30 second crescendo and decrescendos.
If only buying one, it's an easy pick: Go Pro. But, the Strings bundle is so compelling, I expect I'll be glad to have Strings I if only for marcatos and slurred legatos. And NKS.