Two replaceable sticks are faster, but you won't notice.

Not by much. Maybe 5-10% in memory-heavy tasks, and zero in everyday browsing or office work. The soldered + replaceable setup runs in “asymmetric dual-channel” — the matching portion runs in dual-channel, the rest in single. It’s fine.

Most people never push their RAM hard enough to care. If you’re editing 4K video or running simulations, sure, get the full dual-channel. Otherwise, the convenience of an upgradeable soldered stick (like on many ultrabooks) is worth more than the slight speed bump.

You’ll bottleneck on the CPU or GPU long before this matters.