Two identical soldered chips beat one soldered plus one SO-DIMM for performance.

Yes, generally. Two identical soldered chips run in true dual-channel mode, which gives the memory controller full bandwidth. One soldered chip plus one SO-DIMM can work in dual-channel too, but only if the capacities and speeds match exactly. If they don’t, you drop to single-channel (or flex mode, half the bandwidth). Real-world difference depends on your workload — gaming and video editing benefit noticeably; web browsing won’t care.

The tricky part is that many laptops with soldered RAM force you to use a SO-DIMM that isn’t a perfect match. Manufacturers rarely list all the timings, so you often end up with a mismatched pair. If you’re buying new, two identical sticks (even if soldered) are the safer bet for peak performance. If you’re upgrading a soldered system, you’re stuck with whatever SO-DIMM works — just know you’re probably leaving some speed on the table.

Unless you’re doing heavy rendering or pushing high frame rates, the difference is usually small enough to ignore.