Depends on what you care about. If you want are willing to sacrifice a bit of performance for longer battery life and lower weight, then the XPS 13 wins handily. If on the other hand you're willing to sacrifice (quite a lot of) battery life and accept (a lot) heavier weight in exchange for performance, then System 76 will sell you some of the fastest laptops money can buy.
Mobility is relative. They may want desktop level performance that can be moved more easily than lugging around a tower/monitor/keyboard/mouse. Not everyone needs (or cares about) the ability to work away from a power source for more than a few hours.
Mobility is relative. Back when I split my time between two offices and occasionally worked from home I would have loved to have a decently powerful workstation that I could easily transport in a backpack.
