1. The SSD size.
2. The size of the machine. Smaller costs more.
Check reviews about what SSD is used. If the usual M.2 then you can shop for more storage later.
I am thinking of buying either an Ideapad 720s (13", core i7 8th Gen, 8GB Ram, 256 GB SSD) or a Thinkpad x280 (12", core i7 8th Gen, 8GB Ram, 512 GB SSD), and I believe they both have Thunderbolt 3. So I was wondering if there are significant differences between them. Where I am, the Ideapad is around the $1,000 mark, while the Thinkpad is about $400 more expensive. Is the Thinkpad really that much better, and if so, how?