The fair place to meet is Las Vegas, NV — the city closest to the midpoint of Salt Lake City and San Diego. From the farther side that’s about 7 hr 36 min of driving.
Recommended midpoint
Las Vegas, NV
From Salt Lake City
7 hr 36 min
363 mi to Las Vegas
From San Diego
5 hr 33 min
265 mi to Las Vegas
San Diego has the shorter trip; the split is off by about 2 hr 3 min. The alternatives below can even it out. Salt Lake City and San Diego are about 627 miles apart.
Salt Lake City and San Diego are about 627 miles apart by road. Split the difference and you arrive near Las Vegas, the city closest to the halfway point between them. That puts roughly 7 hr 36 min of driving on the Salt Lake City side and 5 hr 33 min on the San Diego side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Las Vegas suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Las Vegas doesn't have what you're after, Phoenix and Los Angeles are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Salt Lake City and San Diego.
Las Vegas, colloquially shortened to Vegas, is the most populous city in the U.S. state of Nevada and the county seat of Clark County. It is the 24th-most populous city in the United States, with 641,903 residents at the 2020 census. Read more →
Salt Lake City, often shortened to Salt Lake or SLC, is the capital and most populous city of the U.S. state of Utah. It is the county seat of Salt Lake County, the most populous county in the state. Read more →
San Diego is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. It is the eighth-most populous city in the U.S. Read more →
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.
Estimates use straight-line distance and typical road speeds; real drive times vary with route and traffic.